-eval { $d->mid_insert('a@b') };
-ok($@, 'error raised when attempting duplicate message ID');
-
+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 $next = (sort(keys %num2mid))[-1];
+is($d->mid_insert('ok@unique'), $next + 1,
+ 'got expected num after failing mid_insert');