Cause: The signature generation algorithm generates signatures that are are 16 bytes in length so it is highly unlikely that any 2 signatures will be identical. This message is raised in such a rare case. See the solution below.
Error solution: Either re-issue the statement that led to the outline being created with some whitespace added or force the outline to be created in a different category.
Database version: Oracle 11g Release 2 (11.2)