If I have a potion that gives 15% resistance to something will taking two give me 30% resistance or is that just a waste of potions ?.

It's a waste. The effects of potions don't stack. Only the highest effect of a potion remains.

If you for example first drink a 15% resistance, and then 20%, you get 20%.

Cool thanks for the help.

Wait, doesn't it just extend the time the potion effects you?

But I think I'm right if you have a weapon or apparel that has the same effect then that stacks. Right? So 15% resistance with equipment that has 15% resistance means you get 30%.