Jamieson, Fausset and Brown on 1 Corinthians 11:28

28. examine--Greek, "prove" or "test" his own state of mind in respect to Christ's death, and his capability of "discerning the Lord's body" (1Co 11:29, 31). Not auricular confession to a priest, but self-examination is necessary.

so--after due self-examination.

of ... of--In 1Co 11:27, where the receiving was unworthily, the expression was, "eat this bread, drink ... cup" without "of." Here the "of" implies due circumspection in communicating [Bengel].

let him eat--His self-examination is not in order that he may stay away, but that he may eat, that is, communicate.