Common mistakes
The second and third radicals are the same letter. They merge: رَدَّ: and come apart again before a consonant ending: رَدَدْتُ.
شَدَدَ → شَدَّ Past: heشَدَدَتْ → شَدَّتْ Past: sheشَدَدُوا → شَدُّوا Past: they (m.)يَشْدُدُ → يَشُدُّ Present: he