Problem Definition

User A assigned a ticket to himself and begins working on it
User B sees the ticket is opened because JIRA doesn't refresh the dashboard quick enough, so he assigns the ticket to himself
Now 2 people are spending time on something because neither of them know that they're both working on it or researching it.

Suggested Solution

A Short pop up or warning message to notify the user who try to assign an issue that has been assign to other users will be helpful in avoiding multiple users working on same issue.