Can't find this via Google or forum search.
For about a month, Acrobat X Pro has been duplicating pages and appending the dupe set to the original page set. I first noticed this when saving out a version of a multi-page PDF in optimized format. But has happened recently when I just combined a group of single-page PDF files into multi-page format. Very frustrating in a production environment where I can't keep stopping to make sure our expensive software did its job without adding errors.