An item with the same key has already been added

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I've added new features on a Dev server and everything works as expected. Now I want to deploy it to a Test server, but I keep getting the error message:
"An item with the same key has already been added." What could be causing this? Even when I try to deploy a dictionary where I haven't changed anything, I still get this error. So I really can't deploy anything anymore...

Thanks in advance for the help.
Best regards, Mario