Common mistakes
The second and third radicals are the same letter. They merge: رَدَّ: and come apart again before a consonant ending: رَدَدْتُ.
اِسْتَمْرَرَ → اِسْتَمَرَّ Past: heاِسْتَمْرَرَتْ → اِسْتَمَرَّتْ Past: sheاِسْتَمْرَرُوا → اِسْتَمَرُّوا Past: they (m.)يَسْتَمْرِرُ → يَسْتَمِرُّ Present: he