code: private void addUniqueConstraintForTableBasedSequence(IdSourceKeyMetadata generatorKeyMetadata) {
  		Label generatorKeyLabel = Label.label( generatorKeyMetadata.getName() );
  		if ( isMissingUniqueConstraint( generatorKeyLabel ) ) {
  			neo4jDb.schema().constraintFor( generatorKeyLabel ).assertPropertyIsUnique( generatorKeyMetadata.getKeyColumnName() ).create();
  		}
  	}
docs: Adds a unique constraint to make sure that each node of the same "sequence table" is unique.