diff --git a/synapse/push/httppusher.py b/synapse/push/httppusher.py
index 38b758e6a..b3b11c5f4 100644
--- a/synapse/push/httppusher.py
+++ b/synapse/push/httppusher.py
@@ -112,6 +112,8 @@ class HttpPusher(object):
 
     @defer.inlineCallbacks
     def _process(self):
+        if self.processing:
+            return
         try:
             self.processing = True
             yield self._unsafe_process()