Duplicate entries

Hello,

I am seeing duplicate entries on a Tungsten m550, in calendar, contacts,
todo and notes. Each Sync seems to add one more identical entry on both
sides, at the moment I have up to 4 of them. Deleting all but one entry
on one side and sync'ing does not really help, I have two entries on
this side and one more on the other side afterwards.

Syncing worked for several months now; no idea what caused this problem.
How can it be fixed?

Thank you very much,
Andreas