Fix same resource file published more than once

This may still happen, though, in low memory situations or very big sites, but I'm not sure it's worth spending time on fixing that. Writing the same file more than once isn't harmful, the negative effect is the false path warning.
 We may find a way to detect that situation if this becomes a real problem.

Fixes #13164

testscripts/commands/hugo__path-warnings_issue13164.txt

@@ -0,0 +1,15 @@
+hugo --printPathWarnings
+
+! stderr 'Duplicate target paths'
+
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+-- assets/foo.txt --
+foo
+-- layouts/index.html --
+A: {{ (resources.Get "foo.txt").RelPermalink }}
+B: {{ (resources.GetMatch "foo.txt").RelPermalink }}
+C: {{ (index (resources.Match "foo.txt") 0).RelPermalink }}
+D: {{ (index (resources.ByType "text") 0).RelPermalink }}
+-- layouts/unused/single.html --
+{{ .Title }}