We still get into situations where we have duplicates by GID/RID in the same collection.
We have to:
* have a way to resolve this if the situation is already existing (preferably without akonadictl fsck)
* Avoid that this can ever happen (in AKAPPEND and MERGE I think).
* Figure out what currently triggers this and resolve the root cause (might be easier to find once AKAPPEND/MERGE properly fails jobs that trigger the problem).