diff --git a/marathon_lb.py b/marathon_lb.py
index a8584c41..60857eb7 100755
--- a/marathon_lb.py
+++ b/marathon_lb.py
@@ -126,7 +126,7 @@ class MarathonService(object):
     def __init__(self, appId, servicePort, healthCheck, strictMode):
         self.appId = appId
         self.servicePort = servicePort
-        self.backends = set()
+        self.backends = {}
         self.hostname = None
         self.proxypath = None
         self.revproxypath = None
@@ -156,7 +156,21 @@ def __init__(self, appId, servicePort, healthCheck, strictMode):
                 self.mode = 'http'
 
     def add_backend(self, host, ip, port, draining):
-        self.backends.add(MarathonBackend(host, ip, port, draining))
+        """
+        A backend server is unique by host, ip and port, not include draining
+        property. When backend servers repeated, the draining property should
+        be true if any server is draining.
+        """
+        backend = MarathonBackend(host, ip, port, draining)
+        if backend in self.backends:
+            oldBackend = self.backends[backend]
+            msg_fmt = ("backend server(%s, %s, %d, %s) duplicated, exists"
+                       "draining status is: %s")
+            logger.warning(msg_fmt, host, ip, port, draining,
+                           oldBackend.draining)
+            oldBackend.draining = oldBackend.draining or draining
+        else:
+            self.backends[backend] = backend
 
     def __hash__(self):
         return hash(self.servicePort)