I think the best policy in cases like this is to contact the officer who mistakenly approved the second waymark, and ask him/her to contact the person who posted the second one, explaining the error and that it is being reconsidered and denied.
Something similar happened - I approved a waymark that had some issues that I didn't catch, and the group leader contacted me so I could deal directly with the waymark, since it was my mistake.
In general, I think it best not to override another officer - in general. There are times a group leader may need to take more direct "police" action, but this should be the last resort, IMHO.
Good issue to raise!