It occurs when a Mim (مَ مِ مُ) with haraka comes after a Mim with sukoon (مْ )or a Noon with harakah (نَ نِ نُ ) comes after a Noon with sukoon ( نْ ). In such a case, the letters Mim (م) and Noon (ن) are pronounced with shaddah and with ghunnah (nasal sound). This is called “idghaam mislayn with ghunnah”.
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