yeah unfortunately ao3 doesn't allow collection moderators to limit claimants per prompt so it's on people to read the rules for themselves until submissions open.

if you're the 2nd+ fill for a prompt we'll reject the fill and ask you to resubmit it to the collection unattached to the prompt. so i guess technically we're allowing multiple fills/prompt (just only one that officially fills it) because we don't want to turn away anyone that wanted to write gmg in good faith. that goes for if you're passing on prompts and writing your own idea and it coincidentally shares similarities with a prompt as well. that's 100% fine and your fic is welcome!

we have this rule in place though because we want to encourage people to fill different prompts! if ao3 would let us enforce 1 claim/prompt we would, because we want as many different prompts filled as is reasonable. so please, if you see a prompt's already been claimed, look elsewhere.