Emphatic commands create special sound combinations involving nūns and number markers. The feminine-plural nūn al-niswah remains, and a separating alif stands before the heavy emphatic nūn. The dual alif remains as well.
Word / formMeaning / explanation
اِفْعَلْنَ → اِفْعَلْنَانِّyou women must certainly do.
لِيَفْعَلْنَ → لِيَفْعَلْنَانِّlet those women certainly do.
اِفْعَلَا → اِفْعَلَانِّyou two must certainly do.
Compare masculine-plural اِفْعَلُنَّ: feminine plural, masculine plural and dual do not have identical endings.