Go to the Domain Manager in Godaddy, choose your domain, click Manage DNS at the bottom.
Now under the Name Server section, click Change.
With Custom chosen, it should show all the nameservers you list below...
Click the 3 dots beside the nameservers you want to remove (bottom 3).
Give it a few hours and your problem should be solved.
The mystery is that there are only 2 addresses in the Godaddy dns manager list, but when I check through the terminal "$ whois duets.eu", it shows 5:
You will probably need to contact Customer Support to resolve this - it sounds like a glitch...
If you want to try something yourself to solve maybe this would work.
Temporarily - You could try to add the name servers it is reporting. (maybe even when you click add you will be able to see these 3 others). Save the change. Give it 10 minutes or so to update in the GD system, then remove the 3 you added.
There is a small chance that might resolve this "bug"