sometimes, although not in every case, my emails get jumbled in terms of there being four or five people all in the same thread when in actual fact they should not be connected at all.
MacBook Pro 15″, macOS 10.14
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this can happen if your Contacts accidentally has the other person's email address assigned to the wrong person.
I.e. you might have John.smith's email under Jerry.Smith's contact info.
Open your Contacts App to verify each email address is in fact the correct one.