Why is the word unstoppable spelled with two

You're right! The word "unstoppable" is spelled with two "p"s. There's no real rhyme or reason for it, it's just the way the word evolved.

It's similar to other words with double letters like "happy" and "pretty". These words were originally spelled with a single "p" but the double "p" became the standard spelling over time. It's a quirk of English spelling!