Oracle, unique constraint violated on Hibernate insert

Question

I'm new to hibernate. I use hibernate 3.3.1 & oracle DB. I generate Id for a table with this annotation :

 @GeneratedValue(generator="carddata-seq-gen")
@org.hibernate.annotations.GenericGenerator(name = "carddata-seq-gen", strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator",
        parameters = {
                @org.hibernate.annotations.Parameter(name = "optimizer", value = "pooled"),
                @org.hibernate.annotations.Parameter(name = "increment_size", value = "100"),
                @org.hibernate.annotations.Parameter(name = "sequence_name", value = "carddata_seq")
                        })

but unfortunately each time server is restarted, this error occurs and last for many seconds:

org.hibernate.exception.ConstraintViolationException: could not insert: [de.shapan.imp.ThirdPartyData]
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:94)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2295)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2688)
at org.hibernate.action.EntityInsertAction.execute(EntityInsertAction.java:79)
at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:279)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:263)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:167)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
at org.hibernate.event.def.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:64)
at org.hibernate.impl.SessionImpl.autoFlushIfRequired(SessionImpl.java:996)
at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1141)
at org.hibernate.impl.QueryImpl.list(QueryImpl.java:102)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)

Caused by: java.sql.SQLException: ORA-00001: unique constraint (FW.SYS_C0025551) violated

at oracle.jdbc.driver.DatabaseError.throwSqlException(DatabaseError.java:113)
at oracle.jdbc.driver.T4CTTIoer.processError(T4CTTIoer.java:331)
at oracle.jdbc.driver.T4CTTIoer.processError(T4CTTIoer.java:288)
at oracle.jdbc.driver.T4C8Oall.receive(T4C8Oall.java:754)
at oracle.jdbc.driver.T4CPreparedStatement.doOall8(T4CPreparedStatement.java:219)
at oracle.jdbc.driver.T4CPreparedStatement.executeForRows(T4CPreparedStatement.java:972)
at oracle.jdbc.driver.OracleStatement.doExecuteWithTimeout(OracleStatement.java:1192)
at oracle.jdbc.driver.OraclePreparedStatement.executeInternal(OraclePreparedStatement.java:3415)
at oracle.jdbc.driver.OraclePreparedStatement.executeUpdate(OraclePreparedStatement.java:3499)
at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:105)
at org.hibernate.jdbc.NonBatchingBatcher.addToBatch(NonBatchingBatcher.java:46)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2275)
... 20 more

I'm so confused. please help me.