Duplicate items are not supported by the "Resources" parameter

This is an Error Message that I have encountered for the first time today.
It is most likely to occur when you copy forms and rename the copy automatically,
which will cause the original to be renamed, too.

In my first project I split the code of a Form into many partial classes, one partial class for each
tab in the form. I created new partial classes the copy & paste way, which worked alright before, but
today I got the Message

The item [...].resources was specified more than once in the "Resources" parameter.
Duplicate items are not supported by the "Resources" parameter

Actually you can only have one Resources File per class. What I did was unknowingly copy the form including the resources file
(...that the VS project explorer hides under the Form's node), so that the resources were double-linked.

Solution: delete the second resource file.

BTW: As of now, I will also try to make my posts googleable for developers with a german IDE by
adding German search terms or error messages:

Das [...].resources-Element wurde im Resources-Parameter mehrfach angegeben.
Doppelte Elemente werden vom Resources-Parameter nicht unterstützt.