Ensure only one client is managed for given window

lib/uh/wm/manager.rb

@@ -46,7 +46,7 @@ module Uh
       end
 
       def manage window
-        return if window.override_redirect?
+        return if window.override_redirect? || client_for(window)
         @clients << client = Client.new(window)
         @events.emit :manage, args: client
       end
@@ -83,6 +83,10 @@ module Uh
         manage event.window
       end
 
+      def client_for window
+        @clients.find { |e| e.window == window }
+      end
+
       def check_other_wm!
         Display.on_error { fail OtherWMRunningError }
         @display.listen_events INPUT_MASK