ok i am playing through as me 550 lifepoint assassin and i decided to get snake skin so i joined the serpent temple cult and when i finaly come to xydusas room she was there and in the
hallway there was another one so i vanish past the first one, she disapeers and now theres the second one so i jump over her go down the passage flip the lever and jump over her again so i continue down the hallway and look back shes gone again so i continue down. so now i come down the hallway to the first elavater room and theres another, so i jump over her,

is this sopused to happen or a bug?

Sounds like a glitch. There's only supposed to be one Xydusa. Hopefully, when you get back into the game, this will be cleared up.

I had a similar thing recently in Skull Castle. The guards in front of G'Ezzered Ra's room were also standing on the next floor up...

That sounds most peculiar!!! But it is not bad... If the bug does not also affect your possibility of getting the snakeskin trait so that you have to reload you will get the experience points from killing two Nagas.

Yes, it must be a bug. I have experienced two Xydusas, too. The "second Xydusa" does not attack and she is immune to everything.
My explanation is that the "second Xydusa" is the one which you have seen on your entering the temple. She has been supposed to crawl through the passage to some point but - the obstacles in the passage caused that she haven't reached the point. She is still here. Simultaneously, the "real Xydusa" has been spawned after your jumping down to the Xydusa lair.
The game uses duplicate characters on other places, too. For example, there are two S'Kesser Da and two Priskielas in the Boogre Prison. One S'Keser Da will curse you, another S'Keser Da is sleeping in her lair. One Priskiela will send you to the "cage", another Priskiela is waiting in the foreroom to S'Keser Da.
What to do? You cannot do much. Jump over her somehow. It's usually possible. The "second Xydusa" will ignore you completely. She will neither attack nor respond to your attemtps to attack.