i3-dmenu-desktop: ignore duplicate files and directories

This is required when e.g. mpv is installed in the NixOS global system
environment *and* in the user environment.

Standalone reproducer:

mkdir -p test1/share/applications test2/share
ln -svf /usr/share/applications/i3.desktop test1/share/applications
ln -svf $PWD/test1/share/applications test2/share
export XDG_DATA_DIRS='test1/share:test2/share'
i3-dmenu-desktop

fixes #4522

i3-dmenu-desktop

@@ -154,6 +154,8 @@ find(
         },
         no_chdir => 1,
         follow_fast => 1,
+        # Ignore any duplicate files and directories and proceed normally:
+        follow_skip => 2,
     },
     @searchdirs
 );