When creating users with email connection via Auth0 java sdk, we found there is no duplication check from Auth0, so there are some users with the same emails in our system.
Hi @hantsy 
Auth0 supports multiple connection type definitions per Auth0 tenant - i.e. Google, Facebook, LinkedIn, etc - including multiple Database (read email/password) connection type definitions. And Auth0 treats each of these as separate connection instances. So whilst Auth0 prevents email duplication within a connection - at least for a Database connection - it does not prevent email duplication across connections; this is by design, otherwise a user would not be able to easily bring multiple identities to Auth0.
I’m not sure what your implementation looks like, but depending on what your application does, and consequently what it is expecting, you may need to leverage Auth0 User Account Linking in order to prevent email duplication within your application.
Hope this helps, but feel free to reach out if you have any follow-up questions 