Verse 1 already said "Barchi Nafshi"! Why is this repeated?
Refer to 103:1:3:1-3.
Radak: Verse 1 blesses Hashem according to His grandeur and Kedushah. Our verse says to bless Him for the benefits that He does for you.
Malbim: This is for the Chesed that Hashem does for the person himself.
Why does it say "[v'Al Tishkechi] Kol Gemulav"?
Rashi, from Brachos 10a: David sang Shirah for nursing. "Kol Gemulav" refers to putting the breasts over [the heart,] the place of understanding. Malbim - Teshuvah nurtures from Binah - "u'Levavo Yavin v'Shav v'Rafa Lo" (Yeshayah 6:10). One opinion said that Hashem arranged thusly lest the baby look at the Ervah - this corresponds to Teshuvah from eyes that pursue heresy. The other said, lest he nurse from a place of filth; this corresponds to Teshuvah from sins due to lust.
Radak: They are all the benefits that He bestowed on you.
Radak citing Shocher Tov: If one puts the mouth of a pouch facing down, the coins inside will fall out. A woman's 'mouth' (opening of the womb) is below, and she walks, and the fetus is guarded in her innards!
Malbim: It is what Hashem does amidst His great love of the Nefesh. Via this, He accepts it in Teshuvah and forgives its sin, like the Mizmor explains after this.
