Error: 'The create unique index statement terminated because a duplicate key was found for the object name dbo.orc_TaskRelations'

Full error message when trying to upgrade Orchestrate:
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​An unexpected error occurred. Please report this problem to your system administrator with the following information:
System.Data.SqlClient.SqlException: 'the create unique index statement terminated because a duplicate key was found for the object name dbo.orc_TaskRelations and the index name IX_orc_TaskRelations. The duplicate key value is (123,456,78). The statement has been terminated.

To resolve this issue Support will need to run some scripts on the database. Therefore please raise a new case online and reference the title of this article and please include details of the old version being upgraded from and the new version trying to be upgraded to.