[sbndrf]

Here is what it does:

1st) When syncing photos from a Mac's iPhoto library to the PlayBook it converts iPhoto events to categories
2nd) It copies all the photos back to the Mac, i.e., imports them to iPhoto, as content of categories (= iPhoto folders) named like the original event

So as a result all pictures are duplicated ...


Edit:
After some further digging I found out that all the re-synced pictures are added into an event with the name "date"-"PlayBook's name". Deleting this event and emptying the trash deletes all the duplicate pictures. Deleting the folders that appeared in the left didn't do the job.

[sbndrf]

Update:
After wondering what fills up my PlayBook, I found out that there were two subfolders in /photos/"MacBook's name"/ called Events and Albums. Both contained copies of the pictures I wanted to sync.