Fix bug 1604. deque.__init__() did not clear existing contents like list.__init__. Not a backport candidate.
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index e5c3218..9efbc27 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -843,6 +843,7 @@
 		}
 	}
 	deque->maxlen = maxlen;
+	deque_clear(deque);
 	if (iterable != NULL) {
 		PyObject *rv = deque_extend(deque, iterable);
 		if (rv == NULL)