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.)