Why was the command to remember God repeated in this āyah?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The repetition is for emphasis on the command to remember Him.
2. The second mention of remembrance is connected with something that was not mentioned in connection with the first, making its repetition appropriate. The meaning of this second instance is: "Remember Him by affirming His Oneness, just as He remembered you with His guidance."
3. The repetition is to indicate constancy, meaning: "Remember Him again and again."

These first three explanations were mentioned by Muḥammad ibn Qāsim al-Naḥwī (i.e. Ibn al-Anbārī).

1. The first mention of remembrance in "remember Allah by the Sacred Monument" refers to the Maghrib and ‘Ishāʾ prayers, which are combined at Muzdalifah. The second mention of remembrance in "remember Him as He has guided you" refers to the remembrance performed while standing in Muzdalifah at morning time the next day. [via Qāḍī Abū Yaʿlā]