Usually, when we see two letters put together it is to define the pronunciation or to differentiate synonyms or just its foreign origins. But some words don't seem to have any reason to double up.
Why does vacuum have two of the letter U?
Why does aardvark have two of the letter A?
Why does llama have two of the letter L?
Because that is the way they are spelled in their original languages, before being borrowed into English. Vacuum is from Latin, Aardvark from Afrikaans/'South African Dutch', and Llama from Spanish. You might have heard that English is an eclectic language; these three words are a good example.
See etymology online:
