Hmmm, just to say "Im always using Duolingo." on its own doesn't sound right. You would have to give it another time qualifier to limit the continuous activity and show that you didn't mean literally "all the time", e.g. "I'm always using Duolingo on the bus in the morning". You can say "I always use Duolingo" on its own and it doesn't express that you are doing it continuously.