Why was "واستفزز" written with two zays in Surat Al-Isra, when linguistically it would become a single shaddah-ed zay?

"Istafzaz" is an imperative form of istafazza. It is permissible for it to undergo both fakk al-idgham (unmerging) and idgham (merging), similar to a doubled verb. Idgham is unmerged if a pronoun of rafʿ (nominative case) that causes its end to be silent is attached to the verb (e.g., halalta - you untied), or if a jussive particle precedes it (e.g., lam yahlul - he did not untie). It is permissible to merge a jussive verb (e.g., lam yahill - he did not untie), which is the dialect of Banu Tamim, while unmerging it is the dialect of the people of Hijaz. In the imperative, both unmerging (uhlul - untie!) and merging (hill - untie!) are permissible. For this reason, "man yartadid" (whoever apostatizes) was recited with two variations: with unmerging and with merging.