Global Navigation links "hydra" themselves

I have a bit of a "hydra" problem. As my attachment shows, I have a drop-down list of subsites in Global Navigation, and a lot of duplicate links to "Waste Water" and "Water Works." Waste Water and Water Works exist only once each.

When I first encountered this duplication issue, there was only one duplicate of each link. When I clicked Edit Links and tried to hide each duplicate link from the drop-down, 2 more copies of the link I'd tried to hide would appear--hence, a "hydra" phenomenon. I have literally no idea what caused this. I did not add multiple copies of each link, and we do not have any custom code in this site collection. I've checked the sites in question and didn't notice anything irregular with the navigation settings there. Has anyone had a problem like this before/any ideas on what to do to remove the duplicates? Help is appreciated!

NEW INFO: Apparently, hiding/unhiding any links in the other drop-down lists in this global navigation will cause the Waste Water/Water Works links to double as well.