Add code for merging duplicates

[alanfgh]

See the list of requirements.

[alanfgh]

@trang recently added #424, which covers the same ground but adds information from sysko. In effect, we need to do some "duplicate-merging duplicate-merging". Although this ticket is older, I'm going to mark it a duplicate of the newer ticket. I'll also add the link to the list of requirements to the newer ticket.