1 files changed, 4 insertions, 0 deletions
diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index 7a04beb57..4d638ea1d 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -517,6 +517,10 @@ class ColumnCollection(util.OrderedProperties):
             # columns collection
 
             existing = self[key]
+
+            if existing is value:
+                return
+
             if not existing.shares_lineage(value):
                 util.warn('Column %r on table %r being replaced by '
                           '%r, which has the same key.  Consider '