What could cause Visual Studio / C# error MSB3105: Duplicate resources

Did you try showing all files in the Solution Explorer? You could have a duplicate .rsx file somewhere in there.

I found the answer in .NET forum posting by Roy Green, and Theresa was right after all, though I did not recognize it.
If you have your main form class split up into partial classes, the partial sections end up in the solution explorer as separate items. And if you double click on them they show up in the designer mode as a normal form. But if you (accidentally) drop a control on these forms, Visual Studio creates a new .resx file and a InitializeComponent routine for it. But since this form is actually just part of the Main Form class it leads to the 'duplicate resources' error. And there is no other solution but to remove the InitializeComponent routine and delete the .resx file by hand.

Be sure that under yourForm.cs no duplicate resources are defined (.resx). If you renamed your Form, remove the old resource because the new one during compile will be created with the new name.