whats in deeyvatu

Ok im claiming deeyavatu ignorance, pls answer my Question someone cause im not hindu?
Deeyavatu means marriage, right?
So when one is married can he by law marry another when the wife is alive?
If yes, Does this mean gulaal is once again a widow and has to wear white?
I Mean doesnt there have to be a divorce first.

I Know this is all fiction but they a really making a mockery of marriage!