The genealogies and lists of names are sometimes very hard to follow in the Old Testament. In this instance, there are two names spelled exactly the same in two consecutive verses. At the end of each verse, they both list the number of sons -- so they can't be the same person?
Belonging to the Gershonites:
Ladan and Shimei.
8 The sons of Ladan:
Jehiel the first, Zetham and Joel—three in all.
9 The sons of Shimei:
Shelomoth, Haziel and Haran—three in all.
These were the heads of the families of Ladan.
10 And the sons of Shimei:
Jahath, Ziza,[a] Jeush and Beriah.
These were the sons of Shimei—four in all.
11 Jahath was the first and Ziza the second, but Jeush and Beriah did not have many sons; so they were counted as one family with one assignment.