ضَرَبْتُ زَيْدًا: “Zayd” is the direct object—the person struck.
ضَرَبْتُ ضَرْبًا: “ḍarban” is the absolute object—emphasis on striking.
ضَرَبْتُ زَيْدًا ضَرْبًا: both may co-occur with different functions.
Identical accusative endings do not imply identical grammatical roles.