[Core] Avoid showing duplicated files in project tree

[rodrmoya]

There are some cases where, it seems, we don't evaluate conditions as we should, resulting in duplicated items being added to the project and, thus, also shown in the project tree (see aspnet/AspNetCore#17088).

Also, by just adding a , duplicated files show also.

So, due to this limitation on the project system, work around it on the project tree, by checking for duplicates based on the files' paths.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1005277

[rodrmoya]

@monojenkins backport release-8.4

[rodrmoya]

@monojenkins backport release-8.4

[mrward]

Alternative fix here which should address the underlying problem of Conditions being ignored on the Content items - #9407

[rodrmoya]

Closing this one then