1 files changed, 11 insertions, 0 deletions
diff --git a/lib/sqlalchemy/dialects/postgresql/dml.py b/lib/sqlalchemy/dialects/postgresql/dml.py
index 76dfafd04..bff61e173 100644
--- a/lib/sqlalchemy/dialects/postgresql/dml.py
+++ b/lib/sqlalchemy/dialects/postgresql/dml.py
@@ -10,6 +10,7 @@ from ... import util
 from ...sql import coercions
 from ...sql import roles
 from ...sql import schema
+from ...sql.base import _exclusive_against
 from ...sql.base import _generative
 from ...sql.dml import Insert as StandardInsert
 from ...sql.elements import ClauseElement
@@ -50,7 +51,16 @@ class Insert(StandardInsert):
         """
         return alias(self.table, name="excluded").columns
 
+    _on_conflict_exclusive = _exclusive_against(
+        "_post_values_clause",
+        msgs={
+            "_post_values_clause": "This Insert construct already has "
+            "an ON CONFLICT clause established"
+        },
+    )
+
     @_generative
+    @_on_conflict_exclusive
     def on_conflict_do_update(
         self,
         constraint=None,
@@ -117,6 +127,7 @@ class Insert(StandardInsert):
         )
 
     @_generative
+    @_on_conflict_exclusive
     def on_conflict_do_nothing(
         self, constraint=None, index_elements=None, index_where=None
     ):