What do you mean? In principle, seed phrases can have any multiple of 3 as number of words. Nami exclusively generates 24 and can import 15 or 24. Other wallet apps can also do 12 and 27.
They are chosen from a list of 2048 words. A word can occur multiple times in a seed phrase, but that is rare with randomly generated phrases due to the sheer amount of possibilities.
There is a checksum function baked into it, so that not every combination of words from this list is a valid seed phrase.
Yes, in other threads it was often, but not always resolved by another seed phrase found somewhere, remembering that it was actually a Ledger account, finding a subaccount, âŚ
There is nothing known about Nami ever generating/displaying wrong seed phrases. It had some other issues during the months, but nothing related to that part. A lot of people have successfully restored/imported their Nami wallets in Nami or in other wallet apps.