kovidgoyal/calibre — pull request #21 by hakan42 (2013-06-25 17:50:00+00:00)
Title: On book merge, merge identifiers as well. In case of conflict, the target wins.

As discussed in http://www.mobileread.com/forums/showthread.php?t=216366 , the identifiers of books are merged too. If a value already exists in the target, the first value therein is retained.