If the final hamzah is the feminine marker of alif mamdudah, it changes to waw when the dual is formed.
صَحْرَاءُ → صَحْرَاوَانِ
حَمْرَاءُ → حَمْرَاوَانِ
Step by step: remove the final hamzah as a feminine marker, put و in its place, and add the dual ending. Thus حَمْرَاءَانِ is not the form taught by this rule.
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Dual and Plural of Mamdud and Maqsur Nouns
If the final hamzah is the feminine marker of alif mamdudah, it changes to waw when the dual is formed.
صَحْرَاءُ → صَحْرَاوَانِ
حَمْرَاءُ → حَمْرَاوَانِ
Step by step: remove the final hamzah as a feminine marker, put و in its place, and add the dual ending. Thus حَمْرَاءَانِ is not the form taught by this rule.