HI Micha!

From NewQSO/WIndow/Contest you can open contest window.
That is a "subwindow" for NewQSO, so you must fill qsos to Contest window, NOT to NewQSO.

First decide if you make a new log for each contest. Do it, open log and contest window and fill contest name and other settings (number needed etc, see help).
Enter qsos via contest window, use NewQSO window just as monitor, or if you need to repair a report or number then take qso back to NewQSO, fix it and save (as usual edit qso goes).

Dupe check will work with new log to every contest as "out of box".
If you use same log for all qsos and want dupe check to work you must go "preferences/fldigi/wsjtx interface/WB4 check starts from" check the "call" checkbox and enter date when contest started.

Remember to uncheck "Call" after contest!