Deskpro tries to match incoming emails to existing tickets based on several criteria, the most important of which are:
Special codes in the body or email headers
Notifications that are sent from Deskpro have special codes embedded in the body and headers that uniquely identify the ticket and are used to confirm that a message should be added to an existing ticket.
If no such codes can be found, Deskpro will fall back on trying to match an existing ticket based on subject.
If there is no ID/REF in the subject, Deskpro will look for tickets with matching subjects, provided the user who sent the message is already on the ticket as the owner or a CCed user.
This prevents two users from replying to each other's separate tickets. But if the two users are CCed on the same ticket, their replies will be collected in the same ticket.
Note that, by default, Deskpro ignores it when a user cc:s an agent, and if an agent replies after being cc:d, a new ticket will be created. You can set your helpdesk to process user CC:s of agents, adding the agent to the ticket as a follower.
If you think there is a mistake in Deskpro's email matching algorithms, you can contact email@example.com with details and we will work with you to figure out what is happening. Please provide the ticket debug logs for both messages: https://support.deskpro.com/en/kb/articles/how-do-i-generate-debug-information-for-a-ticket