It's Easier To See Things in The Library

Throughout Season 5 memory was by far the predominant theme, offering many different takes on what it is, what it does, how memory shapes our perception of reality, etc. Memory though also plays a key critical role in the initial source story - Silence in The Library/Forest of The Dead.

As with everything else, go back to there. The Library is like the playground Ms Evangelista insisted on meeting Donna at inside CAL.

Things are simply easier to see there. 

The Persistence of Memory, Concept of Self and Questions of Identity.

The idea of the the of a self-aware impression of a person being left upon an external device is first introduced in Silence in The Library - the idea of a DATA-Ghost. This is then further expanded to incorporate the idea of a persons whole concious mind being able to be stored and exist digitally on a more massive external hard-drive - an idea that is reintroduced in The Bells of Saint John in the form of the Great Intelligences i-cloud network. 

Season 6 takes these idea's and expand them further - first with the introduction of the memory editing/memory-proof Silence moving on to explore the idea of the idea of duplication during Season 6's mid two-parter The Rebel Flesh/The Almost People - even culminating in the actuality of a flesh duplicate, imbued with the conciousness and memories of a dead human, living on the life of the dead person not just simply in that characters stead, but actually as the deceased individual.

In fact, at one point or another since Steven Moffats Doctor Who began - all of the principal lead characters have, at one point or another have existed as duplicates, each believing them self to be perfectly real and genuine: 

• Rory returned from non existence during Season 5's The Pandorica Opens as an Nestene Duplicate.
• Amy spent pretty much all the first half of Season 6 as a Flesh duplicate.
• The Doctor equally was duplicated by the Flesh during The Rebel Flesh/The Almost People.
• Amy, The Doctor and River were all, at one point or the other, duplicated by the Tesselector. 
• Amy is also accidentally duplicated in The Girl Who Waited  - a story which, again, plays with concepts of self and identity. 

Steven Moffats Doctor Who even kicks off in The 11th Hour with a story featuring escaped  alien convict Prisoner Zero physically passing itself off as various residents of Amy's home village of Leadworth. 

Idea's concerning identity, duplication, concepts of self, and memory have been introduced and explored throughout Steven Moffats entire run - forming by far the most commonly occurring themes.