Two registered letters and one tracking number

I received a failed delivery notice in my letterbox. The tracking number was RR712292897CZ. I took this notice to the post office and collected a registered letter. At home I looked at what had been sent to me — tracking said it came from Prague. I wasn't expecting anything. When I opened it I found it was from my former workplace. Why would they send it from Prague?!

I got my answer a few days later. A new failed delivery notice — same tracking number! That struck me as very odd. I went to the post office intending to give them a hard time. The lady searched the system and couldn't find it. Being thorough, she went out the back and searched… couldn't find it there either, so she checked the complaints pile. And there it was! A parcel with tracking number RR712292897CZ!

The first one, RR948360271CZ, had never been notified to the addressee. The fact that they handed it over at the counter against a notice belonging to a different letter is interesting. At that point, both parcels were already at the post office. What's even more interesting is that parcel RR948360271CZ was close to its storage expiry date.

I don't know where the mistake was made — why one notice never reached me, and why I received two parcels against a single notice. In the end it doesn't matter. I have both letters.