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What is the clearest structural difference between اِحْمَرَّ and اِدْهَامَّ in this level?

Derived Trilateral (with Hamzah) - Bab 5: If'eelaal (Idheemam)

• اِدْهَامَّ has an added internal Alif before the doubled final letter
• اِحْمَرَّ has no Shaddah
• اِدْهَامَّ has a Meem prefix
• They are identical in structure