I think a couple of things may be getting confused here: UU = literal 'double u' as in 2 Us in a row. As Madeline points out, this is quite rare, almost never happens. W = the letter pronounced 'double U'. A pretty common occurence.

The only words other than vacuum (which someone mentioned) that I can think of with "uu" is "continuum" and "mu'umu'u" (which is borrowed from Hawai'ian and is frequently spelled without the 'okina i.e. muumuu). It is quite rare! That being said, if there are two identical letters in a row I would say "double [letter name]" (or "two [letter name]'s"). The exception is "uu". I would almost always say "two u's". If I did say "double u" (which I have only done by accident with subsequent clarification, because it is clearer if you avoid this), the stress is different. "W" -- DOUble yu. "UU" -- double YU (the "dou" in "double" is slightly stressed but most of the stress falls on the letter name) Sometimes I repeat the letter twice (e.g. if it is a common word), but I don't do this as often because that could be mistaken as hesitating if you are reading out a name or code. So for most names or a code I would not do this.

Even if two of the same letter appear back-to-back in a word, when you spell the word say each letter individually. For two U's say "U-U." To use Olivia's example of vacuum, say "V-A-C-U-U-M."

Hi Bronny, There are very few words in English that are spelled using "uu" (the most common is vacuum), so it would be quite unusual to have to spell it out. That being said, if you did need to spell it out loud, I would go with "two Us" to make sure people don't mistake it for "w."