All of a sudden two unrelated email messages were merged together in one email! It happened more than once today, and I don't know how

to fix it!

All of a sudden multiple messages are being combined into one, apparently just because they have the same subject.  How do I un-combine them into the separate messages they originally were?  They are separate unrelated messages from different people.  They just don't belong in the same message, just because they use the same subject.

Who fixed something that wasn't broke? and now is!
Not at all happy with this!