Does scansion ever require a synizesis like ŭŭ > ū or with u replaced by another vowel? I am not sure if this should be called synizesis when the two joined vowels have the same quality, but I hope my intention is clear enough. Here is an artificial example of a hexameter verse:
Est suus cuique viro canis et bene doctus amicus.
Every man has his dog and a well educated friend.
To be able to read this as hexameter, one is forced to make the synizesis ŭŭ > ū.
There was a question about confusion between ŭŭ and ū, but this one is about synizesis instead. An answer to the linked question mentions ĭĭt > īt. If someone can provide a line where īt is used and ĭĭt would invalidate the metric, that would make a nice answer, although I prefer spelling with two vowels if possible.