Why does "suppress" have 2 'p's?

Forgive me for thinking that "supress" was correct spelling. All the rules about English spelling I know say that that word should have only one 'p'. Look at the word "supremacy". That has only one 'p'. The etymology of "suppress" is sub + premere ^[wiktionary]. That doesn't even have 'p's at all, let alone two of them. You know what, I'm just going to keep spelling it "supress".