Heavy nūn al-tawkīd strengthens the assertion. The source displays forms such as لَيَقِيَنَّ and corresponding dual, plural and feminine patterns.
PersonIllustrative heavy formSense
3m singularلَيَقِيَنَّHe certainly will protect
3m dualلَيَقِيَانِّThe two certainly will protect
3m pluralلَيَقُنَّThey certainly will protect
3f singularلَتَقِيَنَّShe certainly will protect
2m singularلَتَقِيَنَّYou certainly will protect
The doubled nūn is an emphasis marker, not the ordinary indicative nūn. Root weakening and person suffixes continue to affect the word.