diff --git a/src/Control/Addressbook/Streaming.hs b/src/Control/Addressbook/Streaming.hs
index 5c22554..1a62e53 100644
--- a/src/Control/Addressbook/Streaming.hs
+++ b/src/Control/Addressbook/Streaming.hs
@@ -43,23 +43,26 @@ chunks n = L.unfoldr $ \case
 run :: IO ()
 run = do
   datDir <- fromMaybe "./" <$> lookupEnv "HOME"
+  let datFile = datDir </> ".addressbook.dat"
+  original <- Set.fromList . map LBS.toStrict . lbsLines <$> LBS.readFile datFile
   xs <- LBS.getContents >>= stream
-  let set = F.fold (parMap rseq F.fold (chunks 20 xs))
+  let set = original `Set.union` F.fold (parMap rseq F.fold (chunks 20 xs))
   runResourceT $
     runConduit $
          CL.sourceList (Set.elems set)
       .| C.map (<> "\n")
-      .| CB.sinkFileCautious (datDir </> ".addressbook.dat")
+      .| CB.sinkFileCautious datFile
   where
     separate = \case
       From x -> [x]
       To xs -> F.toList xs
     -- A set of (locally) unique addresses. Composes with parMap
+    lbsLines = LBS.split (fromIntegral $ ord '\n')
     stream :: LBS.ByteString -> IO [Set ByteString]
     stream =
         traverse (unsafeInterleaveIO . parse . LBC.unpack)
       . filter (not . LBS.null)
-      . LBS.split (fromIntegral $ ord '\n')
+      . lbsLines
     parse path =
       runResourceT $
         runConduit $