I removed the rejected match. That also removed the proposed merge. Now you can reject it again.
Added later: When a bug allows a pair of profiles to be involved in two different matches (proposed merge and an unmerged match, proposed merge and a rejected match, two unmerged matches, etc.), the only way to fix it is to remove one of the matches, if you can. Sometimes this requires a sysop. Removing one match will remove all of them.