Add test

tests/queries/0_stateless/02500_remove_redundant_distinct.reference

@@ -464,3 +464,16 @@ Expression ((Projection + (Before ORDER BY + (Projection + Before ORDER BY))))
 1
 
 0
+-- DISTINCT COUNT() with GROUP BY => do _not_ remove DISTINCT
+-- query
+select distinct count() from numbers(10) group by number
+-- explain
+Expression (Projection)
+  Distinct
+    Distinct (Preliminary DISTINCT)
+      Expression (Before ORDER BY)
+        Aggregating
+          Expression (Before GROUP BY)
+            ReadFromStorage (SystemNumbers)
+-- execute
+1

tests/queries/0_stateless/02500_remove_redundant_distinct.sh

@@ -256,3 +256,7 @@ FROM
     GROUP BY a WITH TOTALS
 )"
 run_query "$query"
+
+echo "-- DISTINCT COUNT() with GROUP BY => do _not_ remove DISTINCT"
+query="select distinct count() from numbers(10) group by number"
+run_query "$query"