Whoever has seen me
Whether seeing the Son is seeing the Father himself or seeing the Father's likeness.
John 14:8-11
8Philip said to him, “Lord, show us the Father, and that will be enough for us.”
9Jesus said to him, “Have I been with you such a long time, and do you not know me, Philip? He who has seen me has seen the Father. How do you say, ‘Show us the Father?’
10Don’t you believe that I am in the Father, and the Father in me? The words that I tell you, I speak not from myself; but the Father who lives in me does his works.
11Believe me that I am in the Father, and the Father in me; or else believe me for the very works’ sake.
What each tradition says
Rabbinic / Midrashic
Kohler presents the position as rejecting as incompatible with reason all claims of the Trinity and of Christ's origin. It also remarked that both Christianity and Islam accepted the roots, but not the logical conclusions, of Israel's faith.
Kaufmann Kohler, Christianity in its Relation to Judaism (Jewish Encyclopedia, 1901-1906) — Section 'Medieval Jewish Views of Christianity' (on Judah ha-Levi's Cuzari)
Patristic
Augustine says that the one who sees the Son sees the Father also. This is not because the Son is the Father, but because the Son in no respect disagrees with the likeness of the Father.
Augustine of Hippo, Tractates on the Gospel of John — Tractate 70 (on John 14:7-10), § 2
Catholic
Haydock says whoever sees Christ sees one not merely a man but, by divine nature, one and the same with the Father. Whoever believes and as it were sees or knows by faith who Christ is cannot but know he is one with the eternal Father—not one person, but one in nature and substance.
George Leo Haydock (Witham notes), George Haydock's Catholic Bible Commentary (1859), on John 14:9 — Comment on John 14:9 (one and the same with the Father in nature and substance)
Orthodox
John of Damascus says some statements about Christ declare the union of nature and the identity in essence with the Father. He cites “I and My Father are one” as an example of this.
John of Damascus, An Exact Exposition of the Orthodox Faith (NPNF2-09, ed. Schaff) — Book IV, Chapter XVIII ('Regarding the things said concerning Christ')
Reformed / Protestant
Gill says seeing Christ is not with bodily eyes but with the eyes of understanding, beholding the Godhead’s perfections in him. Whoever thus sees him has seen the Father, since the same perfections are in both. Christ is the very image of the Father and possesses the same nature, attributes, and glory, so seeing one is seeing the other.
John Gill, John Gill's Exposition of the Whole Bible, on John 14:9 — Exposition of John 14:9 (the Godhead's perfections beheld in Christ)
Evangelical
Albert Barnes says that seeing the Father cannot mean seeing God's essence or substance, since God is invisible and no one has seen him in that way. When God is said to be seen, it means some manifestation or exhibition of him has been made, so we may learn his character, will, and plans.
Albert Barnes, Albert Barnes' Bible Commentary, on John 14:9 — Notes on John 14:9 ('hath seen the Father')