An agent account was cc'd in on the email which created a ticket. But when the agent replies, a new ticket is created.
If a user CCs an agent into an email, by default the agent is not added to the ticket. In this situation, if the agent hits "reply all" to the user's original message, the helpdesk will treat the agent's email as a new ticket.
If you want to change this functionality and have the helpdesk pay attention to user CCs of agents, enable this option from the admin interface, in Tickets > Settings under the Email Settings section.
When this is enabled, an agent CC'd by a user will be added to the ticket as a follower.
Bear in mind the implications of letting users add agents to tickets. Being a follower means that the agent will be able to see the ticket despite any ticket or department permissions that would otherwise prevent it. Depending on their permission settings, an agent may be able to carry out actions on a followed ticket that they would not otherwise. If you want to restrict agent access to some tickets, it may be better not to enable this option.