duplicates on notmuch search not working

[Jeff Templon]

>From the command line:

$ notmuch search --output=files id:Pine.LNX.4.40.0112181809230.6149-100000 at lxplus047.cern.ch
/Users/templon/Maildir/Zar/VMMail/wp2/cur/1528204696.000011.mbox:2,S
/Users/templon/Maildir/Zar/VMdb/cur/1528291801.000080.mbox:2,S
/Users/templon/Maildir/Nikhef/Archives/2001/cur/1528719639.11684_49.simeto,U=13:2,S
/Users/templon/Maildir/Nikhef/Archives/2001/cur/1537863882.18992_50.simeto,U=73:2,S
/Users/templon/Maildir/Nikhef/Archives/2001/cur/1537866158.M994429000P19452Q34R610c5f16.simeto.nikhef.nl,U=133:2,
$ notmuch search --output=files \
     id:Pine.LNX.4.40.0112181809230.6149-100000 at lxplus047.cern.ch \
     --duplicate=2

[ empty output ]

what gives?

JT