Where does duplicate files come from in Visual Studio?

Question

visual studio

Hello.
Here's the focus. In VS, for some reason, 1 library out of 3 is duplicated. And there are no errors on the links.
The files open fine. and VS thinks it's the same file, it doesn't even duplicate the window in the interface.
In a Git folder, everything is in a single instance.
I rebuilt the project, cleaned it, deleted it and reattached it.
It's probably coming from somewhere in the metadata. Just to know where to look.

I ask experts to throw ideas.

Answer 1

The issue was decided by accident.
It turned out that during merge, duplicated metadata was merged into the UML_Logic_Library.cspro file 2 times. VS read them 2 times and did not swear, I began to swear when I started dragging API methods from this library into my project.
I read it and saw that there are 2 headings

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
...