If the final hamzah of an ism mamdud is an original root letter, it remains when the dual is formed.
اِبْتِدَاءٌ → اِبْتِدَاءَانِ
The hamzah remains, and the nominative dual ending انِ is added.
رَفَّاءٌ → رَفَّاءَانِ
Again, the hamzah is retained before the dual ending.
Recognition point: original hamzah is not replaced by waw in this rule.
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Dual and Plural of Mamdud and Maqsur Nouns
If the final hamzah of an ism mamdud is an original root letter, it remains when the dual is formed.
اِبْتِدَاءٌ → اِبْتِدَاءَانِ
The hamzah remains, and the nominative dual ending انِ is added.
رَفَّاءٌ → رَفَّاءَانِ
Again, the hamzah is retained before the dual ending.
Recognition point: original hamzah is not replaced by waw in this rule.