Peresh... Sheresh occur nowhere else.
Ulam and Rakem (Rekem) were probably sons of the elder, Peresh, whose line would naturally be continued, as usual.
Related Commentaries of 1 Chronicles 7:16
1 Chronicles 7:16
16 And Maachah the wife of Machir bare a son, and she called his name Peresh; and the name of his brother was Sheresh; and his sons were Ulam and Rakem.