How was iī pronounced?

Question

Most of the time, Latin doesn't allow two instances of the same vowel next to each other: forms like *mee (from meus) are replaced with alternatives like mī.

However, in I-stem second nouns, the combination iī can occur (for example, filiī "sons"). Do we know how this was pronounced? Was there a consonant separating the two vowels, or did they merge into an extra-long i, or did the quality difference between the two vowels keep them apart?

I'm most interested in Classical pronunciation, but anything pre-Ecclesiastic is relevant!

(Related: this question covers uu, and the accepted answer mentions that a vowel before another vowel seems to have become long-like in quality, but doesn't cover groups like iī where the second vowel is long by nature.)

Answer 1

I don't think we can know for certain, but the fact that the spelling "filii" occurred makes it likely that the word was perceived as having three syllables, and was so treated in poetry. There are various possibilities of how it was actually pronounced:

1) A single extra-long vowel, [i::], in which case the spelling represents perception only.

2) It's possible there may have occurred some subphonemic glide between the vowels, such as [iji:], or (less likely in my opinion) a subphonemic glottal stop, [iʔi:].

3) It's possible there may have been some tonal difference between the two syllables.

4) There's also the fact that short /i/ is generally thought to have had a relatively open pronunciation, i.e., [ɪ], so the pronunciation [ɪi:] may have been the case. (Of course, [ɪ] may have been used in possibilities (2) and (3) above, too.)

I don't think it's possible to know which of these was the case, lacking the evidence of a trained phonetician in Roman times or a time machine. But, I think these are all possibilities.