ORA-26518: push queue synchronization error detected
Cause: Client tried to repush a transaction has already been committed at the master site. A common cause of this problem is an error at the local site in initializing or updating the local site transaction sequence mechanism
Action: Verify that transaction data that RepAPI was attempting to repushed to the master site exists at the master table and is valid and consistent with the local site. If this error occurs, redundantly identified transactions are ignored and then purged from the local updatable materialized view logs. Check that the local site is correctly assigning new transactionIDs and is not accidently generating non-unique values
https://support.oracle.com/
http://www.oracle.com/