QueueAddToProject causes CustomFieldCodeValueAlreadyUsed

Hello

I have a fresh new ProjectDataSet, there I add some new tasks and some
values for several task enterprise custom fields. Those custom fields
are either text fields or fields with lookup tables.

Now when I call ProjectWebSvc.QueueAddToProject() method with that
ProjectDataSet then I get a SoapException with the ProjectServerError
"CustomFieldCodeValueAlreadyUsed".

I don't have any clue what "CustomFieldCodeValueAlreadyUsed" could
mean, does anybody of you know what is meant by this...

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