It is a broken-plural shape in which the plural alif is followed by two letters, as in دَوَابُّ and مَسَاجِدُ, or by three letters whose middle letter is sakin, as in مَسَاكِينُ. In دَوَابُّ, the shaddah represents two letters. The decision depends on the form of the word, not merely on its meaning as “more than one.”
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What Is Sighat Muntaha al-Jumu'?
Cause of Ghayr Munsarif: Sighat Muntaha al-Jumu'
It is a broken-plural shape in which the plural alif is followed by two letters, as in دَوَابُّ and مَسَاجِدُ, or by three letters whose middle letter is sakin, as in مَسَاكِينُ. In دَوَابُّ, the shaddah represents two letters. The decision depends on the form of the word, not merely on its meaning as “more than one.”