Prevent duplicate listing of users for respective scope

The `group(:id)` is faster than `distinct`.

Author: MrSerth

app/policies/external_user_policy.rb

@@ -25,7 +25,7 @@ def resolve
         @scope.joins(:study_group_memberships)
           .where(study_group_memberships: {
             study_group_id: @user.study_group_ids_as_teacher,
-          })
+          }).group(:id)
       else
         @scope.none
       end

app/policies/internal_user_policy.rb

@@ -21,7 +21,7 @@ def resolve
         @scope.joins(:study_group_memberships)
           .where(study_group_memberships: {
             study_group_id: @user.study_group_ids_as_teacher,
-          })
+          }).group(:id)
       else
         @scope.none
       end

app/policies/programming_group_policy.rb

@@ -32,7 +32,7 @@ def resolve
         @scope.joins(:submissions)
           .where(submissions: {
             study_group_id: @user.study_group_ids_as_teacher,
-          }).distinct
+          }).group(:id)
       else
         @scope.none
       end