Duplicated replies to the customer

I'm facing the following problem: agent replies to a customer's claim by "reply" and changes the status of the ticket. Then after some random time (once it's 10 minutes, other times almost 1 day) the very same reply is being sent to the customer. For some tickets it happens up to 3-4 times. It happens regardless of the further reply from the client (some of the agent's answers for tickets are being replied again by the clients, but not all). 

It concerns around 1 in 20-40 tickets, but only 1 agent (at least only 1 mentions it).

Let me know if you need more details.

All best,

Tommy

using:

- Chrome