رَجُلَيْنِ ≠ مُسْلِمِينَ
Dual has fatḥah before yāʾ; sound masculine plural has kasrah before yāʾ.
The preceding vowel helps distinguish the two forms.
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Why Fatḥah Before Dual Yāʾ
The Dual Noun: Inflection, Maqṣūr, Mamdūd and Iḍāfah
رَجُلَيْنِ ≠ مُسْلِمِينَ
Dual has fatḥah before yāʾ; sound masculine plural has kasrah before yāʾ.
The preceding vowel helps distinguish the two forms.