What is the possible cause of receiving duplicate emails for the same ticket number

[Glenn]

Our client received duplicate emails for the same ticket number. Upon triggering the alert sent button from our side.

[Iago Docando]

There could be litterally dozens of different reasons.

To name one: You had an automatic notification configured in your Jira Server and on top of that you're running Jira Service Desk, wich has its own notification system.

Of course there's no way to know based on the information you have provided but you should check everything related to notification schemes and any automations afecting the project at hand.

Best of luck.