I would say that it is the GM's call as to whether or not they can select the same thing twice -- the thing to remember is that a x2 and a x2 doesn't equal x4, but only x3. for those Boons that work in that manner.
As for the Snags, again, the GM should make the final call (after discussing it with players), but for the most part, it is the player's choice (just like Boons). While most will go for the lower cost ones, there will be the occasional player who does take the bigger one. (especially if you rule that you only receive the effects of 1 Snag per round, meaning that taking 2 or 3 1 point Snags means that they will take a Snag effect for each of 2 or 3 rounds in succession, as opposed to getting it all over with at once).