diff --git a/qubes/devices.py b/qubes/devices.py
index 21b8c7b3..2a641ca2 100644
--- a/qubes/devices.py
+++ b/qubes/devices.py
@@ -71,6 +71,7 @@ class DeviceAssignment(object): # pylint: disable=too-few-public-methods
         return "[%s]:%s" % (self.backend_domain, self.ident)
 
     def __hash__(self):
+        # it's important to use the same hash as DeviceInfo
         return hash((self.backend_domain, self.ident))
 
     def __eq__(self, other):
@@ -234,21 +235,21 @@ class DeviceCollection(object):
 
         devices = self._vm.fire_event('device-list-attached:' + self._class,
             persistent=persistent)
-        result = []
+        result = set()
         for dev, options in devices:
             if dev in self._set and not persistent:
                 continue
             elif dev in self._set:
-                result.append(self._set.get(dev))
+                result.add(self._set.get(dev))
             elif dev not in self._set and persistent:
                 continue
             else:
-                result.append(
+                result.add(
                     DeviceAssignment(backend_domain=dev.backend_domain,
                                      ident=dev.ident, options=options,
                                      frontend_domain=self._vm))
-        if persistent is not False and not result:
-            result.extend(self._set)
+        if persistent is not False:
+            result.update(self._set)
         return result
 
     def available(self):
@@ -335,7 +336,7 @@ class DeviceInfo(object):
                 setattr(self, group, dev_match.group(group))
 
     def __hash__(self):
-        return hash(self.ident)
+        return hash((self.backend_domain, self.ident))
 
     def __eq__(self, other):
         return (
diff --git a/qubes/tests/devices.py b/qubes/tests/devices.py
index 86a3926a..540e03ef 100644
--- a/qubes/tests/devices.py
+++ b/qubes/tests/devices.py
@@ -111,7 +111,7 @@ class TC_00_DeviceCollection(qubes.tests.QubesTestCase):
     def test_011_double_attach(self):
         self.collection.attach(self.assignment)
 
-        with self.assertRaises(LookupError):
+        with self.assertRaises(qubes.devices.DeviceAlreadyAttached):
             self.collection.attach(self.assignment)
 
     def test_012_double_detach(self):