I noticed another duplicate and this time I remembered what I have done. On Tuesday evening at about 8 pm I created an appointment on my phone for 6pm on 1.2. A little bit later I modified that appointment to start at 7 pm. And now I see two appointments in Groupwise (at 6pm and at 7pm) but only one appointment on the phone.
The original entry should not have synced with groupwise before the change, because my peak-time stops at 5pm and after that syncing occurs every 12 hours. I can't be sure about that, though.
So, this leads me to wonder if there are some (known) problems with E71's calendaring?