وَأُدْغِمَتِ الْيَاءُ فِي الْيَاءِ.
Translation: Then one yāʾ was assimilated into the other yāʾ.
Idghām merges two like letters into a doubled consonant. The shadda on the yāʾ in مُسْلِمِيَّ represents this assimilation.
This is not assimilation of the plural nūn; that nūn had already been removed by iḍāfa.
Opening interactive controls...
Noun Case Class 9 — Sound Masculine Plural with My
وَأُدْغِمَتِ الْيَاءُ فِي الْيَاءِ.
Translation: Then one yāʾ was assimilated into the other yāʾ.
Idghām merges two like letters into a doubled consonant. The shadda on the yāʾ in مُسْلِمِيَّ represents this assimilation.
This is not assimilation of the plural nūn; that nūn had already been removed by iḍāfa.