Running user add twice results in a corrupt database

[swarris]

Hi,

By running the user add command:
java -jar opensilex.jar --BASE_DIRECTORY=$PWD --CONFIG_FILE="../config/opensilex.yml" user add --admin
twice, the application is broken.

"result": {
      "title": "Unexpected internal error - org.opensilex.sparql.exceptions.SPARQLException",
      "message": "Multiple objects for some unique property",
      "stack": [
          "org.opensilex.sparql.service.SPARQLService.getByUniquePropertyValue(SPARQLService.java:457)",
          "org.opensilex.sparql.service.SPARQLService.getByUniquePropertyValue(SPARQLService.java:435)",
          "org.opensilex.security.user.dal.UserDAO.getByEmail(UserDAO.java:35)",
          "org.opensilex.security.authentication.api.AuthenticationAPI.authenticate(AuthenticationAPI.java:120)"
      ],
      "fullstack": [
          "org.opensilex.sparql.service.SPARQLService.getByUniquePropertyValue(SPARQLService.java:457)",
          "org.opensilex.sparql.service.SPARQLService.getByUniquePropertyValue(SPARQLService.java:435)",
          "org.opensilex.security.user.dal.UserDAO.getByEmail(UserDAO.java:35)",
          "org.opensilex.security.authentication.api.AuthenticationAPI.authenticate(AuthenticationAPI.java:120)",
          "jdk.internal.reflect.GeneratedMethodAccessor138.invoke(Unknown Source)",
          "java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)",
          "java.base/java.lang.reflect.Method.invoke(Method.java:566)",
....
      ]
  }

I would prefer to get an error message, like 'unable to add user, user already exists.'