We just realized we have doubled a lot of leads and it is causing some issues with outbound efforts. Our issue was we had a list of contacts imported earlier in the summer. 

We just recently uploaded a new list and when they were uploaded for some reason they did not check to see if they were already in the system. These new leads in a lot of cases were assigned to a different owner. 

Is there a way to take all of our leads and check for doubles / lead owners?

Or do I need to go through each one?

Thanks