my $ret = $d->mid_insert('a@b');
is($ret, undef, 'duplicate mid_insert in undef result');
is($d->num_for('a@b'), $mid2num{'a@b'}, 'existing number not clobbered');
my $ret = $d->mid_insert('a@b');
is($ret, undef, 'duplicate mid_insert in undef result');
is($d->num_for('a@b'), $mid2num{'a@b'}, 'existing number not clobbered');