How to solve a "duplicate items" error in the "Resources" parameter

Create a backup of both the .cs and .designer.cs file which is giving the error and then delete that file from the project. Rebuild the solution and then copy back both .cs and .designer.cs files. Finally, rebuild the solution again.

I guess the item "..." is a form, then:

1. Remove the "..." form from recent solution (and save somewhere)
2. Recompile the project
3. Save the solution
4. Restart the solution
5. Add the "..." form
6. Recompile