Foreigners often make some strange mistakes in Chinese when they study the most difficult language--- ---Chinese. We try to analyze typical sentences to correct these errors.

         Wǒ chī le liǎnɡ liǎnɡ mǐfàn.
False: 我吃了两两米饭。
         Wǒ chī le èr liǎnɡ mǐfàn.
True: 我吃了二两米饭。

二用于表示度量衡单位的量词“两”前，因为读音的关系，不能用“两”，只能用“二”。
Before 两 which is also a measure word of weight, 二 rather than 两 is used for pronounciation reason.