When you create a new Windows Application project in SharpDevelop 3.1, the IDE creates a default form called 'MainForm'. If you add an existing form with the same name to this project then build this project, you may get this error:
The item "MainForm.Designer.cs" was specified more than once in the "Sources" parameter. Duplicate items are not supported by the "Sources" parameter. (MSB3105).
SharpDevelop displays a line number (though the line number is a furphy) but not the file with the error. It turns out that when you add the form, the IDE appends another <Compile Include="MainForm.Designer.cs"> entry in the .csproj file. The solution is to delete the duplicate entry. Note that if you use Visual Studio, there is no duplicate entry in the .csproj file.