[SCHEDULE] Detect duplicate IterVar in reorder (#575)

src/schedule/schedule_lang.cc

@@ -259,6 +259,7 @@ Stage& Stage::fuse(IterVar outer, IterVar inner, IterVar* p_target) {  // NOLINT
 }
 
 Stage& Stage::reorder(const Array<IterVar>& order) {  // NOLINT(*)
+  std::unordered_set<IterVar> seen_var;
   StageNode* self = operator->();
   for (IterVar iv : order) {
     CHECK(iv->iter_type == kDataPar ||
@@ -266,6 +267,10 @@ Stage& Stage::reorder(const Array<IterVar>& order) {  // NOLINT(*)
           iv->iter_type == kThreadIndex)
         << "Cannot reorder IterVar("
         << IterVarType2String(iv->iter_type) << ")";
+
+    CHECK_EQ(seen_var.count(iv), 0)
+        << "Same axis can not appear more than once " << iv;
+    seen_var.insert(iv);
   }
   ArrayNode* all_vars = self->all_iter_vars.CopyOnWrite();
   ArrayNode* leaf_vars = self->leaf_iter_vars.CopyOnWrite();

tests/python/unittest/test_lang_schedule.py

@@ -43,6 +43,13 @@ def test_reorder():
     assert tuple(s[T].leaf_iter_vars) != order
     s[T].reorder(*order)
     assert tuple(s[T].leaf_iter_vars) == order
+    try:
+        # pass duplicate IterVar
+        # must raise an error
+        s[T].reorder(xi2, xi1, xi2)
+        assert False
+    except tvm.TVMError:
+        pass
 
 def test_split():
     m = tvm.var('m')