why do I have Unique index or primary key violation error?

Hi

When I run my process in Bonita Studio, Sometimes I have Unique index or primary key violation error. The time of this error is where I want to save an new row in BDM.
with reset engine and run it again, it doesn't work. To fix this error I have to edit PERSISTENCEID of last row in my table !!!
Do you have any idea that what's the reason of the error??

`Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
at org.hibernate.exception.internal.SQLStateConversionDelegate.convert(SQLStateConversionDelegate.java:129)
at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:49)
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:126)
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:112)
at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:211)
at org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:96)
at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:58)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3032)
at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3558)
at org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:98)
at org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:492)
at org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:197)
at org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:181)
at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:216)
at org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:334)
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:289)
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:195)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:126)
at org.hibernate.jpa.event.internal.core.JpaMergeEventListener.saveWithGeneratedId(JpaMergeEventListener.java:73)
at org.hibernate.event.internal.DefaultMergeEventListener.saveTransientEntity(DefaultMergeEventListener.java:271)
at org.hibernate.event.internal.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:251)
at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:189)
at org.hibernate.event.internal.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:85)
at org.hibernate.internal.SessionImpl.fireMerge(SessionImpl.java:876)
at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:858)
at org.hibernate.internal.SessionImpl.merge(SessionImpl.java:863)
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:1196)
... 30 more

Caused by: org.h2.jdbc.JdbcSQLException: Unique index or primary key violation: "PRIMARY_KEY_100 ON PUBLIC.MATRLLABREQUEST(PERSISTENCEID) VALUES ( /* 27 */ 49 )"; SQL statement:
insert into MATRLLABREQUEST (persistenceId, CASE_ID_LBREQ ...... `