With some helpdesks, Agents answer users via email rather than using the Agent Interface. 

In this situation, it would be useful to assign tickets to the first agent to reply.

Here's how to do that with a Trigger:

1. Go to Admin > Business Rules > Triggers > New Reply Triggers and click New to make a new trigger.

2. Add a title for the trigger.

3. Under Event, uncheck everything except By Agent, and Email.

4. Under Criteria, add Agent / is / Unassigned agents.

5. Under Actions, add Set assigned agent / Current agent.

6. Click Save.