This might happen when you deploy another application in the same server environment where the already defined Transaction Id is again attempted to add in the existing table and violates the unique identity constraint. Reset the sequence number.
Other Useful links for the topic in discussion
http://docs.oracle.com/cd/E17904_01/web.1111/b31974/bcstatemgmt.htm#ADFFD1306
http://docs.oracle.com/cd/E17904_01/web.1111/b31974/bcstatemgmt.htm#ADFFD1307
http://docs.oracle.com/cd/E17904_01/core.1111/e10105/repos.htm#ASADM11531