Duplicate Value Error

When trying to merge 2 contacts (one created by Classy and the other internally) we are getting the attached errors. Has anyone seen this and been able to get a fix? I have tried to change the Account to be the same on both of the records as I read somewhere it may be that, but it did not work.

Error: duplicate value found: csp__Unique_Key__c duplicates value on record with id: 0034R0000465LUm

Error: Failed to merge the contacts. Error: ConMergeException:[]: duplicate value found: csp__Unique_Key__c duplicates value on record with id: 0034R0000465LU 

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