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Rom. 7:18 For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
Gal. 2:20 I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
Is there an incongruity in Paul saying that no good thing "dwells" in him, though he has Christ in him? No -- Paul is using a Greco-Roman rhetorical technique; see here for more.
Even so, the objection fails to distinguish between the Greek words involved. The Galatians word is zao, while the Romans word is oikeo. The latter connotes physical occupation. The former connotes influence or substance. ("Man does not live (zao) by bread alone.").