Shutter Island: how knowing the twist of a film before you watch it never does any good...(SPOILER ALERT)

When it comes to film twists in the past, I've managed to keep myself in the dark quite well. It's impressive that I saw Fight Club and The Sixth Sense both for the first time in 2013 and managed to keep myself from hearing the twists for so long after their original release. Before I sat down to watch both films with my parents (both had seen them before me) I had the same warning of "just so you know, there's a twist at the end" and then urge me to guess (bringing to mind the twist-guessing game in series two of The IT Crowd... like Roy, I feel it ruins the fun of the movie.)

But Shutter Island was completely different. The first thing I heard about this film came from my brother, who claimed it "proved DiCaprio didn't die on Titanic, as he wakes up on a beach." Neither is true, but a cute idea he had almost certainly heard from someone who hadn't seen or heard of the film (N.B it is actually apparently The Beach that "proves" this...Hmm.) Before sitting down to watch it with my family I was aware that the ending was going to be something we wouldn't expect, but after reading the short synopsis I found on IMDB I already knew what it was most likely going to be. This might be because I consider myself a bit of a film buff, but may also be because at the time I saw it I was in the process of writing my own script set in a mental institution; similarly, with a twist ending. Writing a twist into a story is actually a lot easier than you would have thought, so my worry was that Scorsese and I had similar ideas on what constitutes as a good twist in a mental health drama. And so, I felt I had gone into watching a film for the first time having spoiled the ending for myself unknowingly.
(TRYING NOT TO INCLUDE SPOILERS BY GIVING A KIND OF VAGUE DESCRIPTION)
In Shutter Island, DiCaprio plays U.S Marshall Edward "Teddy" Daniels sent to a psychiatric facility for the criminally insane on said island to investigate an escaped patient, Rachel Solando. After reading the synopsis, that gave away no more information than the above, I immediately knew that the twist in the ending would be that Daniels was a patient on Shutter Island himself, it was inevitable.
Similarly, the twist ending in the script I was in the process of writing was exactly the same. A medicine student shadows an esteemed psychiatrist in an institution, only for it later to be revealed that this is the hallucinations and ideas coming from a brain-damaged girl, in a coma after attempting suicide by hanging and damaging her nervous system.
Credit to Scorsese, he must have had an idea that when it comes to mental health dramas there are only a select few twist endings that are effective. He took one we probably would have suspected first, but developed it, with the use of characters Rachel Solando and Andrew Laeddis and the use of the "law of four" and the question of who is "patient 67"?