Duplicate Key Error on Plugin

I've deployed a Dynamics CRM 2015 plugin that migrates activities from the Opportunity entity to the Case entity. Whenever I test the assembly, I get an error that says:

Unhandled Exception: Microsoft.Xrm.Sdk.InvalidPluginExecutionException: Cannot insert duplicate key.

I'm trying to see what the cause may be. I've made sure the name of the assembly is different than the previous assembly that was in CRM, and have also commented out the GUID that is in the AssemblyInfo.cs class file. Any ideas on what this may be? Thanks.