Here is the full list of all 18 Happiness Double Room achievements.
Ending 1 Achieved
Ending 2 Achieved
Ending 3 Achieved
Sleeping with Sister Minami Achieved
Shower with Sister Minami Achieved
Flirting with Sister Minami Achieved
Bathroom H Event Achieved
Enter Free Play Mode
Vigilance Level never increased during a round of the game.
Happiness Level reached 100
The Vigilance Level decreased to 0
Find the duckling and click on it
Happy Times 1 Achieved
Happy Times 2 Achieved
Happy Times 3 Achieved
Happy Times 4 Achieved
♡ Minami ♡
All other achievements unlocked