Ashley

It's not つ that doubles the following letters, it's small tsu (っ), which is used primarily for this purpose. So つも is not mmo but tsumo - mmo is っも. Incidentally you can only use small tsu after another letter, because in Japanese you can only have double letters in the middle of words. Incidentally sometimes small tsu is placed at the end of utterances, especially in things like manga, but this is a less formal convention and indicates a glottal stop, which occurs at the end of some emphasised utterances in Japanese.