I was on another forum and they were discussing this. I thought it was impossible, but they brought up some interesting theories that really got me. Does anyone here think that they are the same being, somehow? 

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Seifer was Edea's knight, and in an alternate reality or maybe a timeline untainted by Ultimecia messing with it from the future Squall would have been Rinoa's knight when she was hated by the world. In fact he flat out says that he'd be her knight.
In the final boss fight, second phase, you fight against "Griever" or whatever you named Squall's little pendant thing. This is not a coincidence. Why would a final boss fight, a monster apparently summoned or created by Ultimecia have a connection to Squall? A bit odd, is it not?
Can you make an argument as to why Ultimecia tries to kill her supposed "past self" then? Particularly the junction scene when Rinoa is junctioned to Adel?
She did it to herself...when the party beats her at time compression it caused people to fear sorceress power...thusly in her time people caused her much strife and persicution based on her being a sorceress so she lived in ...fear I guess of fearing her death by the White Seed's simply because of a'Monster" thet would try and destroy the world,wich was a sorceress...herselfwell, I don't really know or care despite all the points I've up.
I can understand how one person can change to be that evil though. Women with their constant moodswings and such. If period pain make them that pissed then imagine how they'll be being all alone in a dark castle.