ls "<mountpoint>/origins/<invalidurl>" creates two empty directory entries with the same name

$ swh fs mount /tmp/foobar
$ ls -l /tmp/foobar/origin/ 
total 0
$ ls -l /tmp/foobar/origin/aaa
total 0
$ ls -l /tmp/foobar/origin/   
total 0
dr-xr-xr-x 1 dev dev 0 Dec  2 14:40 aaa
dr-xr-xr-x 1 dev dev 0 Dec  2 14:40 aaa

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Migrated from T2839 (view on Phabricator)