لَمْ نَلْقَ غَيْرَكَ إِنْسَانًا يُلَاذُ بِهِ / فَلَا بَرِحْتَ لِعَيْنِ الدَّهْرِ إِنْسَانًا
We found no person besides you to seek refuge in; may you remain the pupil of Time’s eye.
Mutamathil occurs when the two fully matching words belong to the same grammatical class. The pair “insānan / insānan” uses nouns in both places, but one means a human being and the other the pupil of an eye.
Their spelling and pronunciation coincide while their senses do not. The effect depends on recognizing that second sense, not on treating the line as simple repetition.