diff --git a/csharp/App/Backend/DataTypes/Methods/Session.cs b/csharp/App/Backend/DataTypes/Methods/Session.cs
index 52683e16f..7d626a218 100644
--- a/csharp/App/Backend/DataTypes/Methods/Session.cs
+++ b/csharp/App/Backend/DataTypes/Methods/Session.cs
@@ -148,12 +148,13 @@ public static class SessionMethods
             && Db.Delete(installation);
     }
     
-    public static Boolean Create(this Session? session, User? newUser)
+    public static Boolean Create(this Session? session, User newUser)
     {
         var sessionUser = session?.User;
+        var userAlreadyExists = Db.GetUserByEmail(newUser.Email);
 
         return sessionUser is not null
-               && newUser is not null
+               && userAlreadyExists is null
                && sessionUser.HasWriteAccess
                && newUser
                    .WithParent(sessionUser)