Oracle Error Codes

cannot modify string.string more than once in a transaction

An attempt was made to modify an object more than once in a transaction. This error is usually caused by a DDL statement that fires a system trigger that tries to modify the same object. It can also happen when an attempt is made to perform more than one DDL operation on a queue table in the same transaction without issuing a commit between operations.

How to Solve :

Do not create system triggers that might modify an already modified object. Also, do not specify more than one DDL operation on a queue table in the same transaction.