add dup ts check case

tests/system-test/2-query/interp.py

@@ -2907,6 +2907,8 @@ class TDTestCase:
         # add duplicate timestamp for different child tables
         tdSql.execute(f"insert into {dbname}.{ctbname1} values ('2020-02-01 00:00:15', 15, 15, 15, 15, 15.0, 15.0, true, 'varchar', 'nchar')")
 
+        tdSql.query(f"select _irowts, _isfilled, interp(c0) from {dbname}.{stbname} range('2020-02-01 00:00:00', '2020-02-01 00:00:14') every(1s) fill(null)")
+        tdSql.error(f"select _irowts, _isfilled, interp(c0) from {dbname}.{stbname} range('2020-02-01 00:00:00', '2020-02-01 00:00:15') every(1s) fill(null)")
         tdSql.error(f"select _irowts, _isfilled, interp(c0) from {dbname}.{stbname} range('2020-02-01 00:00:00', '2020-02-01 00:00:18') every(1s) fill(null)")
         tdSql.query(f"select _irowts, _isfilled, interp(c0) from {dbname}.{stbname} partition by tbname range('2020-02-01 00:00:00', '2020-02-01 00:00:18') every(1s) fill(null)")