Everytime they save the world they do something differently but ultimately end up with the same ending which is Ulti dies and SeeD is created. In one of their trips back Squall could have never made it back and Rinoa could have gone el loco about it. She could have studied time magic and stuff and time compression (which she already knew about thanks to Ulti) and then became Ultimecia kind of.

Another thing I heard is that Squall lived but Rinoa still became evil and Squall stood by her side to protect her (some say she turned him into Griever).

Time loops suck.