Hi Raj,
Ok, so this exception information is copied from ServicePulse and these are actual message failures. I wrongly assumed the exception information was copied from a log file.
The [Unique] attribute is only needed on the properties that you use to correlate sagas to messages. You state that the attribute is already present on PaymentId, could you verify on the SQL Server that this column indeed has an index? If it does not have an index then SQL Server needs to perform a table scan to retrieve the correct record. If it does not have an index then please add a Unique index on the PaymentId column.
Please share any updates.