What, if any, difference is there between long vowels and a double vowels, i.e. consecutive identical vowels? For example, what is the difference between /i:/ and /ii/?
Phonetically, could it be that the former forms one syllable and the latter form two?
Or could there be a change in quality in one of the two vowels? Or a change in pitch, loudness, or duration of one of the two vowels? (I understand that these possibilities are not mutually exclusive.)
Are there languages that have both phonemic vowel length and double vowels?
Sorry for the lack of background, but I haven't found an answer to this question on the net.