Calendar View Remove Duplicate Items
you can try views distinct module
from module page: 


  Relationships or other joins in Views often create "duplicate" results. For example, a node with a field that has multiple values may show up in the View once per value in the multi-value field. It's frustrating, and the "DISTINCT" SQL query option in the Views UI does not actually solve the problem because the result row is technically distinct. This module aims to give a simple GUI method to remove or aggregate these "duplicate" rows.


I hope it helps, be carefull in features, the module settings are not exported in features. If you use features, once you push your code, on production site, set your view UI settings again, it's easy and simple.