Changing capitalization of C# file causes duplicate csproj entry

[iggyvolz]

Godot version:
Godot 3.0 Beta 2

OS/device including version:
Windows 10 Education

Issue description:
My script file is named Root.cs. I accidentally typed root.cs, which caused an entry for root.cs to be added to my .csproj file. Since Windows is case-insensitive, the file is listed twice which causes:

Steps to reproduce:
Create a new Godot mono project. Add a Node2D, and attach a script Root.cs to it. Change the resource path to res://root.cs, and attempt to compile.

Possibly related to #12917.

• I searched the existing GitHub issues for potential duplicates.

[akien-mga]

I'd say it's the same issue as #12917, let's close it as a duplicate and keep track of both symptoms there.