fix #5965 - validate user registration before adding user to a site

Oqtane.Server/Managers/UserManager.cs

@@ -181,14 +181,24 @@ namespace Oqtane.Managers
                 succeeded = true;
                 if (!user.IsAuthenticated)
                 {
+                    // validate if the user already exists for the site
+                    succeeded = string.IsNullOrEmpty(GetUser(user.Username, user.SiteId).Roles);
+                    if (succeeded)
+                    {
+                        // a user is registering for a new site - ensure their password is valid
                         var result = await _identitySignInManager.CheckPasswordSignInAsync(identityuser, user.Password, false);
                         succeeded = result.Succeeded;
                         if (!succeeded)
                         {
-                        errors = "Password Not Valid For User";
+                            errors = "User Already Exists In Installation But Cannot Be Added To A Site Because The Password Provided Is Not Valid";
                         }
                         user.EmailConfirmed = succeeded;
                     }
+                    else
+                    {
+                        errors = "User Already Exists In Site";
+                    }
+                }
             }
 
             if (succeeded)