Albert Barnes Commentary Exodus 38:26

"a beka a head, [that is], half a shekel, after the shekel of the sanctuary, for every one that passed over to them that were numbered, from twenty years old and upward, for six hundred thousand and three thousand and five hundred and fifty men." — Exodus 38:26 (ASV)

A bekah - Literally, “a half”: the words “half a shekel,” etc. appear to be inserted only for emphasis, to enforce the accuracy to be observed in the payment. See Exodus 30:13. Respecting the capitation and the numbering of the people, see Exodus 30:12.