I can understand why people think this. For a long time, I did too. Only, to me it never quite felt right. So I looked a little closer and what I found may surprise you.
Before I go any further, I need to explain a little about the "time loop theory".
Introduction to Lost's "Time Loop Theory" (TLT)
Some believe it, some don't, others don't understand it while many more haven't even heard of it. There is no "one" theory and they're all confusing. The one thing they all have in common is the notion of a time loop where a reset occurs and events re-play slightly differently each time until "The Incident" occurs which breaks the loop cycle and allows the events in 2007 to play out.
Let me break it down to the heart of the TLT that's most relevant. The first thing to keep in mind is the fact that it's only our Losties (including the Candidates) that are time-jumping. Not the Others (remember Locke meets up with them only to time jump himself while the group of Others see him disappear). The idea here is that as the Island jumps around in time, our Losties experience the blooping, and our Candidates become stuck in it (without realising it of course). The Island is then stopped from blooping by John Locke after Ben previously started the blooping by moving it. This is the point at which things kept cycling round and round, and our Losties were (unbeknownst to them) never able to get to 2007 UNTIL the Incident happened (which remember, Jacob himself engineered through Eloise, Ben etc). This in turn meant that the Candidates (previously caught in the loop) could all of a sudden travel FROM 1977 to 2007 (jump out of the time loop) and therefore defeat the MiB who thought the Candidates were all perpetually stuck in the loop.
That is why fLocke looks so shocked when Jacob whispers to him "they're coming". He was referring to the Candidates that the MiB had thought were lost forever looping. When Jacob says "you found your loophole", that had a double meaning, the MiB's loophole and the end of the time loop.
A fellow Lostpedia member has contributed the following which is a more in-depth account of the TLT that fits nicely with my theory.
Here’s the deal, there is a time loop. There is also “Whatever Happens Happens” (WHH) BUT when there is time loop, there is no WHH. The two things contradict each other because WHH is linear. As long as someone living in 2007 goes back to any time before 2007 then WHH is no longer true. If it was, then there would never have been any need for course correction.
A good way to help clear this up is to clearly define what flashbacks are, opposed to time travel. There is time travel for a few Losties (Dharma LaFleur group and the Ajira 316 group) and there are flashbacks (regular flashbacks and Desmond style flashbacks).
These things all fall into the pool together during a time loop when all characters interact somewhere within the loop. A perfect example of this is Desmond and Eloise - because neither Desmond NOR Eloise ever actually physically travel through time. They are not in the loop, they are actually of the fiber of the loop.
Points in Desmond’s life are like a beacon ala Daniel Faraday. If you recall the interaction with Daniel and Desmond in the Hatch, which results in 2007 Desmond waking suddenly with an ADDED experience in his memory. An event that had not previously happened before. Desmond could recognize that his memory had just been altered, because his sense of flashback is somewhat heightened.
BEFORE Desmond turned the fail safe key, his sense of perception was most likely normal, and there were no crossed lines in his history. Turning the key for him turned his linear time line into a folded mirror. It wasn't until that very instant that an alternate bearing of time connected Desmond to Eloise in the past, and due to that, Desmond suddenly remembered it from where he lay on the Island. Physically ‘here and now’ is here and now, but otherwise there is no here and there is no NOW.
What fuels this is the fact that LaFleur and (certain passengers on) Ajira 316 have physically gone into 1977 Dharma. What fuels that is the fact that they were somehow shanghaied back in time by a chain of events that began with the fail safe key, which led back to The Incident that started that chain in the first place. LOOP!
‘Course Correction’ is the correction of the loop. WHH is more or less a moral code of ethic because if time travel is possible, it is no longer true. It is course corrected one small piece at a time. (Wallyp)
The important thing here is not the detail of the theory, but rather, the evidence that strongly suggests a loop is shaping events on Lost.
Desmond is the variable
The one 'variable' amongst a sea of 'constants' is Desmond. He is "uniquely and miraculously special". He is also the key to breaking the loop because his existence appears to transcend time and space somehow. Faraday knew it as did both Eloise and Widmore. (It is no coincidence these 'time keepers' are all directly related). Desmond could also withstand otherwise lethal doses of electromagnetic energy - the very thing that allowed all forms of time travel on Lost (from the zapping of the polar bears and rabbits in the Orchid Station time chamber, to the entire Island blooping through time when the Frozen Donkey Wheel was turned).
I believe Desmond's "visions" are NOT glimpses of the future. They are dim memories of his past. Evidence suggests that while all our Losties are experiencing a time loop, Desmond experiences it slightly differently. Unlike everyone else (who are unaware of the looping), he doesn't remember his "previous" experiences until he approaches the same critical event again. Something he sees/experiences then triggers the deja-vu moment.
It is for this reason that I strongly believe that none of his visions were precognitive. It's important to understand the exact meaning of this word.
pre·cog·ni·tion (noun) Knowledge of a future event or situation, especially through extrasensory means.
"Knowledge of future event or situation" - in other words, something that actually happens in the future before it happens. But evidence suggests that Desmond actually does not see the actual future. A facsimile of it perhaps where many things are the same, but never the exact same thing. Why else are there (usually big) differences between his visions (things that I'm arguing happened on a previous time loop that he is just realising but thinks is the future) and what actually happens (seeing Claire get into the helicopter for example - something that in the timeline we see, never happened, yet Desmond seemed very sure of it). How else do you explain that in all his visions he sees Charlie die, yet he was always saved? (Except of course for that last time - though even that was flawed as he also said it would be Claire that got into the helicopter with Aaron but he was wrong as it was Kate instead). Therefore his visions are not in fact what happens in the future.
Here's the thing. If the visions were truly pre-cognitive, then what happens in those visions MUST be exactly what then happens! If you're seeing the future and then that reality is different when that moment occurs - then you didn't actually see the future in the first place. They can't be different! Some would then argue that by knowing the future you could change it - but then that rescinds the original vision and you have what scientists (and Einstein himself) call a 'temporal paradox' - or a scientific impossibility.
HOWEVER - if the visions/flashes are 'memories' from the 'past' (due to time looping), then you could totally change that vision as happened time and time again when Desmond kept saving Charlie. One vision explicitly showed an arrow to Charlie's neck. The reality was that Desmond moved Charlie out of the way just in time. So the original vision could not under ANY theory be a precognitive flash because what was seen in that vision DID NOT HAPPEN!!! Not in THAT timeline anyway. But he did see it so it must have occurred - and I am suggesting that it was in his past memory.
Charlie regularly guided to danger by Desmond
When you think about it, Charlie was mostly only ever in danger because of Desmond taking him on treks into the jungle etc. He only died because he did what Desmond said he saw in one of his flashes. How else would he have thought of swimming down to the Looking Glass Station and if so, how and why? My guess is that in the previous loop, that's what happened, then only because Desmond said he saw it and told him that his heroics would pay off, did Charlie do it in the latest loop (that we see in which he dies).
This would also explain why Desmond was so confident that Jack fixed Sarah (during their first meeting in the stadium) - he recalled their on-Island conversation where Jack told him that he did, in fact, fix her. It would also explain why the microwave beeping sound reminded him of the Hatch computer as well as many other instances where Desmond 'remembers' something familiar but doesn't quite know why.
By extension - this also means that there were multiple time loops. Exactly how many is unknown. The reason I say this is that - if we accept the visions as being 'memories', then Desmond has experienced multiple "Charlie deaths" (and he can only die once per loop). In each new loop, he saves Charlie's life one extra time - culminating in what happens in "Through The Looking Glass" at the end of Season 3. He made the decision (under some duress from his previous chats with Eloise) to give up saving Charlie because he believed it was fruitless as "time had a way of course correcting". The thing is, this was planted in his head by Eloise so that a pre-determined course of action takes place that leads to the Incident.
Remember also that Desmond had the ability to see the after-life (flash-sideways). He experiences this as he is being zapped by Widmore on the Island towards the end of Season 6. Only, because he doesn't know it's the after-life, he confuses it with an alternate timeline (just like the entire Lost audience did at that point), believing that his actions in helping fLocke and Jack in the Source, will re-set time. Of course he was wrong but there is no doubting his ability to see the FS (afterlife).
(Please note: The image above was not made by me. It comes from timelooptheory.com and is flawed as it was created before the final season aired. However I am using it here simply to give a visual demonstration of how the TLT works in principle).
Desmond's 1996 mind travel
After turning the failsafe key in present Island time, Desmond's consciousness re-entered his 1996's self. At one point during the mind travel, Desmond leaves Widmore's office, disheartened and angered. He then sees a singing Charlie Pace performing on a London street corner just outside, he recognizes him, and he says to himself;
DESMOND: No this, I remember this. This all happened before! ("Flashes Before Your Eyes")
This appears to me to be yet another clue that there is a time loop in play. He said it "all happened before". He didn't say he could see into the future.
Then we have this critical line by Eloise Hawking in Season 5.
Eloise: I came, Penelope, to apologize. Your husband has become a casualty in a conflict that’s bigger than him, that's bigger than any of us.
Penny: What do you mean? Is Des gonna be okay?
Eloise: I don't know. For the first time in a long time, I don't know what’s going to happen next. ("The Variable")
We know much of her knowledge came from her son, Daniel's diary, and this line (that she no longer knew what was going to happen next) is due to the fact that the diary had no further information past a certain date. But this date is significant for another reason. 2007 is the "end date" of the time loop - the one Daniel's idea of detonating the bomb in 1977 effectively ended, by sending the Candidates back to their original time instead of looping back for eternity.
The clue left by Jacob
JACOB: It only ends once. Anything that happens before that is just progress. ("The Incident, Part 1")
On the surface, this line from Jacob is quite straightforward. It's is his affirmation in humanity - his belief that in the end, we get things right. But look a little closer and it's potentially a big clue about the time loop.
"It only ends once" - a reference to the Incident which ends the looping.
"Anything before that is just progress" - a reference to the multiple loops that each time, progress things further so that eventually, the conditions required for the detonation of the hydrogen bomb are facilitated.
Evidence of Jack's time loop
In this video I discovered recently (that I note was made before we had any knowledge that Jack had in fact previously been on the Island in 1977, there is a great deal of evidence that Jack (and to some degree Kate) had some kind of awareness of things that transcended other people's. Careful consideration is given to his actions and words. See for yourself... it's quite revealing. While some of what the video maker is suggesting is a bit of a stretch, they did get it right. Jack had been on the Island before and does indeed show evidence of being in a loop.
Who knew about the time loop?
Well, there's no way to be really sure but based on the available evidence, Eloise and Widmore appeared to know at least something about it - but of course they were being manipulated into ensuring things happened a certain way by either (or both) the MiB and Jacob, not to mention Faraday's diary. If I had to guess, I'd say both knew, but it was Jacob who was able to understand it well enough to use it to his advantage and place him in checkmate position against Smokey.
I am also quite certain that Ben Linus knew of it, and it's all to do with the images on the left. It's the photo that was handed to Sun and Lapidus by ghost-Christian (ie: Smokey) and it was found on the wall of Ben's office, taken in the Dharma days to tell them where the people they were looking for could be found (1977). The fact that this photo was found hanging in clear sight in Ben's office, suggests strongly that Ben (and possibly other people) knew when Jack crashed on the Island in 2004, that he'd been on it before. The photo also features Hurley and Kate. How is it possible that it was in Ben's office hanging on the wall and he not know the whole time that at some point they would time travel back to the Dharma days? Ben and the Others took over all documentation owned by the D.I. so they'd have known of these people's existence prior to the plane crash. Some might say that Smokey/Christian placed it there as the picture was on an angle - a theory which is quite plausible. But the picture frame came from somewhere, so if it was hidden in one of the drawers, it still means Ben knew about it. ("Namaste")
The time looping compassThere is yet another clue that supports the possible existence of a time loop, and it is handed to us on a platter by the writers. ("Follow the Leader"), we once again meet the compass that we first saw being shown to a young Locke by Richard in a previous episode. This compass exists in a self-contained temporal paradox (loop), in that it is never created. Richard gives the compass to a time-shifting Locke, who then returns it to Richard in the 1950's. It never enters the loop nor does it ever exit it either. It just is. ("Because You Left") ("Jughead")
The fact that this little nugget was intentionally written into the story by the LOST producers, I think is quite significant.
Linear time and final words
So as you can see, it all comes down to how you define the visions and whether you believe that time looping is occurring.
Something else that's important; In Lost mythology, the writers are very clear about how time worked. It isn't like it is on Star Trek or Fringe where you could (and often did) have multiple timelines/universes. That's not possible on Lost. Time on Lost was completely linear. You could move forwards and back, and could even loop around several times, but you're still only moving along the same line of time.
The Universe, unfortunately, has a way of course-correcting. That man was supposed to die, that was his path. Just as it's your path to go to the Island. You don't do it because you choose to, Desmond; you do it because you're supposed to. (Eloise Hawking)
Put simply, in Desmond's case "deja vu" doesn't mean a "feeling" that events are repeating themselves - they actually are! As Eloise explained, events must repeat themselves, and it's Desmond's job to make sure they do.
See you in another loop, brother!