May possibly have been mentioned before but there seems to have been a theme of duality/doubles/duplicates in most episodes of this series. Bit like the eye/visual theme in many episodes of S5

TIA/DOTM - two Doctors of different ages
Curse Black Spot - two universes colliding
TDW - not as obvious but there are two versions of the Tardis, the actual physical tardis and Idris
The ganger episodes - lots of flesh duplicates
AGMGTW - Melody Pond and ganger version
LKH - Amy and Teselecta Amy
Night Terrors - Not sure about that one?
TGWW - Amy and Old Amy
CT - Not sure about that one?

Now is it just coincidence or a result of the underlying plots that there are lots of doubles rather than anything deliberate? Or is this duality a key features of S6 and part of the resolution - two Doctors/two universes etc - that has been speculated upon

The episode where they saved young Amy and changed events for the older Amy is the most significant IMO.

Yes.

The thing is, you could probably find some kind of duality in almost any story once you start looking for it, particularly if you're counting things like two universes colliding and Idris and the Tardis. It's easy when you're actively seeking out something... take Closing Time, for example - You could have said real Amy and the poster of Amy.

But I don't think there's any intentional recurring theme present.

Night Terrors had the doll Amy. The Doctor's Wife also had the old Rory in the Tardis corridor. Plus flesh Amy applies to most of the first part of the series.

Yeah, I've thought this for a while. There have been doubles and fake versions of main characters all the way through, apart from in The Black Spot (well, Amy) and in Closing Time. Too many to be coincidence, so I'm positive there's some significance to it. The plot hinged on Amy being duplicated and Melody being switched. Also we were given the possibility of a duplicated Doctor which I'm not discounting as a hint.

I don't for one moment go along with the idea of everyone and his wife being flesh doubles, but I do think we're being prepped to realise that who we see might not be who we think we see..

I've been wondering about this ever since the two-universes-occupying-the-same-space resolution of the Black Spot.