Yes, but it doen’t suppose a problem! The important thing is who did it first and when (identity of who did it and when it was done). Every time something is registered (albeit repeated) a timestamp associated with the identity of the person who made the certification and the certified content is generated.
Musicdibs, being a tool with validity and worldwide scope, can’t know if someone has already certified something before another person in another registration system or another part of the world. As we said, that is not what is relevant, but the hability to prove in a reliable way WHO DID IT BEFORE . And that is what Musicdibs guarantees you.