Emails are being tracked by 2 different cases

Hi all,

CRM 13 on-Premise here. I'm having a problem with an email that is being tracked by two different cases, here's what's happening:

It seems like there were a couple of emails sent regarding a specific case, and here's the final result:

1.- If I check in CRM the email, the subject says that it's regarding Case 1. The regarding field is set to Case 1 as well.

2.-If I click in the regarding lookup, it redirects to Case 2. 

I don't know if maybe this email was sent to a user and that user answered changing the regarding field to Case 2 or something like that, to be honest I don't really know what could happened here.

Any ideas?

Thanks in advance.

Regards.