This Bible contradiction is from the Skeptic's Annotated Bible.
They were the sons of Benjamin
Genesis 46:21 View context
And the sons of Benjamin [were] Belah, and Becher, and Ashbel, Gera, and Naaman, Ehi, and Rosh, Muppim, and Huppim, and Ard.
They were the grandsons of Benjamin
Numbers 26:38-40 View context
The sons of Benjamin after their families: of Bela, the family of the Belaites: of Ashbel, the family of the Ashbelites: of Ahiram, the family of the Ahiramites:
Of Shupham, the family of the Shuphamites: of Hupham, the family of the Huphamites.
And the sons of Bela were Ard and Naaman: [of Ard], the family of the Ardites: [and] of Naaman, the family of the Naamites.