دُنْيَا and بُنْيَان contain nūn sākinah followed by yāʾ inside one word. Here the n stays clear; do not apply the usual cross-word idghām with ghunnah.
Compare مَنْ يَقُولُ, a two-word sequence requiring idghām, with دُنْيَا, a single word requiring clear nūn.
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One-word exception: dunyā and bunyān
Sākin Nūn & Tanwīn — Iẓhār, Idghām, Qalb and Ikhfāʾ
دُنْيَا and بُنْيَان contain nūn sākinah followed by yāʾ inside one word. Here the n stays clear; do not apply the usual cross-word idghām with ghunnah.
Compare مَنْ يَقُولُ, a two-word sequence requiring idghām, with دُنْيَا, a single word requiring clear nūn.