diff --git a/changelog.d/5045.bugfix b/changelog.d/5045.bugfix
new file mode 100644
index 000000000000..483942aad70b
--- /dev/null
+++ b/changelog.d/5045.bugfix
@@ -0,0 +1 @@
+Prevent the abiltiy to upgrade the same room more than once.
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 17628e268438..0d1670167f55 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -76,7 +76,9 @@ def __init__(self, hs):
 
     @defer.inlineCallbacks
     def upgrade_room(self, requester, old_room_id, new_version):
-        """Replace a room with a new room with a different version
+        """Replace a room with a new room with a different version. Will
+        raise an exception if the room has already been upgraded, or in
+        currently in the process of being upgraded.
 
         Args:
             requester (synapse.types.Requester): the user requesting the upgrade
@@ -85,12 +87,22 @@ def upgrade_room(self, requester, old_room_id, new_version):
 
         Returns:
             Deferred[unicode]: the new room id
+
+        Raises:
+            NotFoundError if the room is unknown
+            SynapseError if this room has already been upgraded
         """
         yield self.ratelimit(requester)
 
         user_id = requester.user.to_string()
 
         with (yield self._upgrade_linearizer.queue(old_room_id)):
+            # Check that this room has not already been upgraded
+            tombstone = yield self.store.get_room_tombstone(old_room_id)
+
+            if tombstone is not None:
+                raise SynapseError(400, "This room has already been upgraded")
+
             # start by allocating a new room id
             r = yield self.store.get_room(old_room_id)
             if r is None:
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 0bfe1b4550c6..c183422aa958 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -433,7 +433,8 @@ def get_room_predecessor(self, room_id):
             room_id (str)
 
         Returns:
-            Deferred[unicode|None]: predecessor room id
+            Deferred[unicode|None]: predecessor room id, or None if a
+            predecessor is not found
 
         Raises:
             NotFoundError if the room is unknown
@@ -444,6 +445,33 @@ def get_room_predecessor(self, room_id):
         # Return predecessor if present
         defer.returnValue(create_event.content.get("predecessor", None))
 
+    @defer.inlineCallbacks
+    def get_room_tombstone(self, room_id):
+        """Get the tombstone event of an upgraded room if one exists.
+        Otherwise return None.
+
+        Args:
+            room_id (str)
+
+        Returns:
+            Deferred[FrozenEvent|None]: the tombstone event, or None if a
+            tombstone event was not found
+
+        Raises:
+            NotFoundError if the room is unknown
+        """
+        # Retrieve the room's tombstone event if it exists
+        state_ids = yield self.get_current_state_ids(room_id)
+        tombstone_id = state_ids.get((EventTypes.Tombstone, ""))
+
+        # If we can't find the tombstone, assume we've hit a dead end
+        if not tombstone_id:
+            defer.returnValue(None)
+
+        # Retrieve the room's tombstone event and return
+        tombstone_event = yield self.get_event(tombstone_id)
+        defer.returnValue(tombstone_event)
+
     @defer.inlineCallbacks
     def get_create_event_for_room(self, room_id):
         """Get the create state event for a room.