"Do you know any namesake for Him?" meaning: any equal. That is, you do not know anyone like Him. And "samiyan" (namesake) is from the root meaning: al-musamah (emulation/rivalry).
﴿هَل تعلم لَهُ سميا﴾ أَيْ: مَثَلًا؛ أَيْ: أَنَّكَ لَا تعلمه، و (سميا) هُوَ من: المساماة
