Modified in what way?
As to your problem: Perhaps you’re dealing with a mail thread here, consisting of two messages?
If I select this mail entry, the selection list has two elements. In any case, the problem is not DT here, as it just works with the selection in Apple’s Mail. If that app reports two elements, then that’s that.
(SInce you’re using list view, the small blue right-pointing arrow might be to the left of the subject. Or it’s not visible in dark mode?).
Sounds a bit contrived – why not have Apple’s Mail retrieve the mail from the remote server?