Why do two different emails have the same Reference and In-Reply-To?

Question

postfix

Greetings!
There are absolutely two different letters, not related to each other, but caught in one thread. The reason turned out to be that both letters had the same Reference and In-Reply-To
Piece from Origin:
Letter one:

Content-Type: text/plain; charset=UTF-8;
 format=flowed
Date: Tue, 14 Mar 2017 09:10:14 +0100

Subject: Re: Betreft: my subj1
In-Reply-To: mid:144
References: mid:144
Message-ID: <[email protected]>

Letter 2:

Subject: Re: Betreft rmy subj 2
In-Reply-To: mid:144
References: mid:144
Message-ID: <[email protected]>

This happens extremely rarely, but I would like to understand what could be the reason, and where to dig.