only attempt to create the postgis extension when it does not already exist

django/contrib/gis/db/backends/postgis/creation.py

@@ -89,7 +89,7 @@ class PostGISCreation(DatabaseCreation):
             self.connection.close()
             self.connection.settings_dict["NAME"] = test_database_name
             cursor = self.connection.cursor()
-            cursor.execute("CREATE EXTENSION postgis")
+            cursor.execute("CREATE EXTENSION IF NOT EXISTS postgis")
             cursor.connection.commit()
 
         return test_database_name