$ hg init a
$ hg clone a b
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd a
$ echo '[paths]' >> .hg/hgrc
$ echo 'dupe = ../b' >> .hg/hgrc
$ echo 'expand = $SOMETHING/bar' >> .hg/hgrc
$ hg in dupe
comparing with $TESTTMP/b
no changes found
[1]
$ cd ..
$ hg -R a in dupe
comparing with $TESTTMP/b
no changes found
[1]
$ cd a
$ hg paths
dupe = $TESTTMP/b
expand = $TESTTMP/a/$SOMETHING/bar
$ SOMETHING=foo hg paths
dupe = $TESTTMP/b
expand = $TESTTMP/a/foo/bar
$ SOMETHING=/foo hg paths
dupe = $TESTTMP/b
expand = /foo/bar