Update DotnetExeCompilator.cs - Duplicate Asm fix

[yonixw]

My project has the same DLL in an imported package and from the editor (System.Windows.Forms.dll). So the plugin will fail with this error (for any class):

Error when updating files: 'Script.cs', 
System.Exception: Compiler failed to produce the assembly. 
Output: 'error CS1703: Multiple assemblies with equivalent identity have been imported: 
'E:\Unity\Project\Assets\StandaloneFileBrowser\Plugins\System.Windows.Forms.dll' 
and 'C:\Program Files\Unity\Hub\Editor\2022.3.7f1\Editor\Data\MonoBleedingEdge\lib\mono\unityjit-win32\System.Windows.Forms.dll'.
Remove one of the duplicate references.'

So this fix add a check that no 2 DLLs with the same file name will be added. This solved it for me, and might be useful .