Surya had two wives.
The first one was Samjna - the daughter of Visvakarma. Surya had three children in her - Manu, Yama and Yami. She found Surya's heat unbearable and left him after creating an exact replica of herself called Chhaya. Surya didn't know this. He had another three children in Chhaya - Sani, Savarnamanu and Tapati.
Chhaya showed bias towards her own children. Yama, angry with this , raised his legs to kick her. Chhaya cursed Yama that his legs would fall off. Yama went to his father and complained , who modified the curse that only some portion of the flesh from his legs would fall off. Surya confronted Chhaya , who confessed that she was not the original. Surya sent Chhaya away and brought back Samjna.