Spring Data CrudRepository and Transactions

I'm trying to implement transactions on a CrudRepository Interface. I'm a beginner with this and my current problem is that when receiving a lot of requests from different clients, I'm sometimes getting a duplicate. To avoid that I wanted to use SQL Transactions and their implementation with Spring but I'm unable to get it working.

Here is how I've tried to do it :

@Repository
@EnableTransactionManagement
@Transactional
public interface ApplicationPackageDao extends CrudRepository<ApplicationPackage, Long> {

/**
 * Find if a record exists for this package name ,
 * @param packageName
 * @return
 */
@Transactional
ApplicationPackage findByPackageName(String packageName);

}

However it doesn't seem to work. I tried to add the @Transactionnal annotations earlier in the Java methods I'm calling but I can't get it working either.

How am I supposed to work with transactions on CrudRepository ? Or am I using completely the wrong thing?