SharePoint 2013 List - Managing Duplicate Entries based on 2 columns.

I'm trying to add some script to my list. Basically this is list is for employees to enter information as they get it completed. So a worker will have multiple entries, and for each entry they will choose from a choice column (Titled "Event"). The Event column has 8 choices that the employee will do one at a time over a few months. What I want is to be able to avoid the worker from entering the same choice twice. However, I can't just turn on "Enforce Unique values" on the Event column because each employee will be doing those same events.

I believe I need to use PreSaveAction() somehow to limit the worker from entering the same event twice, but I'm not really sure how to go about this.

Sample Data:

Worker - Event
John - Attend meeting
John - Attend health Screening
Mike - Attend meeting
Mike - Attend health Screening
Mike - Attend meeting

When Mike in this instance would try and save that last record, he should get an error message stating something along the lines of "You have already completed that Event"

Any help would be appreciated, Thanks!