Stopped #3961 from happening

src/Ombi.Schedule/Jobs/Plex/PlexUserImporter.cs

@@ -88,6 +88,12 @@ namespace Ombi.Schedule.Jobs.Plex
                             _log.LogInformation("Could not create Plex user since the have no username, PlexUserId: {0}", plexUser.Id);
                             continue;
                         }
+
+                        if ((plexUser.Email.HasValue()) && await _userManager.FindByEmailAsync(plexUser.Email) != null)
+                        {
+                            _log.LogWarning($"Cannot add user {plexUser.Username} because their email address is already in Ombi, skipping this user");
+                            continue;
+                        }
                         // Create this users
                         // We do not store a password against the user since they will authenticate via Plex
                         var newUser = new OmbiUser

src/Ombi/Controllers/V1/IdentityController.cs

@@ -457,6 +457,10 @@ namespace Ombi.Controllers.V1
             {
                 return Error("You do not have the correct permissions to create this user");
             }
+            if(user.EmailAddress.HasValue() && await UserManager.FindByEmailAsync(user.EmailAddress) != null)
+            {
+                return Error("This email has already been taken");
+            }
             var ombiUser = new OmbiUser
             {
                 Alias = user.Alias,