The light emphatic nūn is sākin rather than doubled. Both forms still express emphasis; removing the shaddah does not turn the construction into an ordinary un-emphasised imperfect.
Word / formMeaning / explanation
لَيَفْعَلَنَّ ← لَيَفْعَلَنْhe will certainly do it.
لَيُفْعَلَنَّ ← لَيُفْعَلَنْit will certainly be done to him.
لَأَفْعَلَنَّ ← لَأَفْعَلَنْI will certainly do it.
But this substitution is not permitted in all fourteen positions. The six positions left blank by the textbook must remain blank.