I don't mind using both the & and / if the fic is clearly exploring both the shippy and gen/friendship aspects of the characters' relationships. But yeah, sometimes people just put in both when they only mean one. I often ship characters who have a strong friendship in canon, and sometimes I actually just want to read about that friendship, and then it's hard to find fics that are actually about that when people tag both just in case!

ETA: How Do I Tag aka a cry for help from many confused users of AO3. :-D

Haha, that's a lot of confusion and consternation! :-D To be fair, I also find it hard to choose the right additional tags to describe my fics beyond the fandom, characters and relationships. Thinking up enticing and accurate tags is hard! But I try to be careful about the more clear-cut tag categories, like is this actually in this fandom.