How to solve a “duplicate items” error in the “Resources” parameter

Question

While working on my project, I got this error:

The item \"...\" was specified more than once in the \"Resources\" parameter. Duplicate items are not

Answer 1

I received this error after I copy and paste Form1 (for example) in my project.I rename Copy in Form2, but Form2 had next line in *.cs "public partial class Form1". As i understand after copying class name didn't change. I resolved this problem by adding a new form and copying there controls from my old form. The Old form was deleted.