This was implemented about two months after it was requested, and apparently never documented. 

A migration will be rejected if...

- ...An *exact*, word-for-word duplicate already exists on the destination site, or...
- ...A question created within a week of the one being migrated exists on the destination site with the same author and *very* similar body text.

Moderators can override this restriction, to allow for the sorts of migrate->close/merge [scenarios Michael describes][1] - however, it is important that they coordinate with moderators on the destination site when doing so to avoid confusion. 


  [1]: http://meta.stackexchange.com/questions/97632/check-to-make-sure-a-duplicate-does-not-exist-before-migrating/97634#97634