cmd/doctor: Remove duplicate source paths from conflicting_urls check

[iBug]

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This is a 🙋 feature or enhancement.

Summary

If a single source file is somehow duplicated in Jekyll's build pipeline, it irritates jekyll doctor to complain about Conflict: The following destination is shared by multiple files, which doesn't make much sense.

This isn't normally possible, but my workaround for a missing feature for the jekyll-feed plugin relies on this. Nevertheless, it's nice to have as an extra sanity check to remove the duplicates.

Context

This PR is one way to fix jekyll/jekyll-feed#390.

[ashmaroli]

Hello @iBug, Since source files do not get duplicated normally, I believe this patch has no place in Jekyll Core. I suggest you move this patch into your workaround plugin.

My stand on this PR is: wont-merge.