Mercury lived in the Moon Kingdom. When that world ended, she was reborn, reincarnated as Mizuno Ami with the power to transform into Mercury.

I'm wondering if it could be said that original Princess Mercury was here before.

Edit: And just saw the above conversation, which explains it well. It's entirely the same character, yes. It shouldn't be appable since she's already here. (It's more like... a very distant past canon point, that she barely remembers hazy things from.)