First thing you must do is achieve at least a Silver Medal on the level Dionaea. After doing this, you will unlock the rubber duck in the "Laboratory" game mode. The next step is to launch the "Laboratory" game mode from the main menu, find and select the rubber duck, and place it at the top of the screen. Simply back out to the main menu after this, and the rubber duck will fall in the background and make a "quack" thus awarding you the achievement.

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