I had this issue.
It happened when an incident had been unresolved and then users attempted to resolve the incident again.
I contacted support and the issue was that the process from unresolve was not linked back to "open", instead it was linked to in progress, or something like that.
Apparently b/c of that, there wasn't a new GUID generated for the unresolved incidnet, and the second resolve was throwing the duplicate key error.
Changing the process to link the unresolve all the way back to open fixed the issue for any newly created or resolved incidents. But it did not fix any existing incidents that had been unresolved.
I would up giving a few trusted analyst the ability to close incidents and that was the workaround for existing incidents, close them rather than resolving them a second time.