I have had two "permanent" unmerged matches. Both are cases of same name, same geography, same time frame, but not clear if it is the same person.
For one of them, once the 1939 register came out, II was able to determine that they were DIFFERENT people, and removed it.
The other one is 17th century, and a common name in that location at that time. I have looked, without luck, for evidence either way. I periodically search to see if any new data has shown up. But, until I find any, it will remain an unmerged match.