What does "Maximum copies per contact exceeded" mean in the mail processing log?

I reviewed the mailing log in the system and saw an error. What does "Maximum copies per contact exceeded" mean?

There is a setting on the account level that will cap the number of duplicate copies that can be received by one recipient. This is set to 20 by default. 

We do not encourage sending more than 20 copies to the same contact. However, if your use case deems it necessary, it is possible to override this default setting when creating the campaign.

In step 3 of the campaign creation process, un-check the three boxes under "Duplicate recipient handling" to prevent the duplicates from being pulled.