This question is based on an older English translation of these texts which does not fully reflect the nuances of the original languages.

First, the Hebrew root which the KJV translates 'covet' in Ex 20:17 is cited by the Brown, Driver, Briggs Hebrew lexicon as 'desire, take pleasure in' as the general root meaning, with the specific definition given below in the bad sense of inordinate, ungoverned, selfish desire. That is, the word has a bad sense, which invariably is the meaning in Ex 20:17's negative prohibition.

Second, the Greek verb used in 1 Cor 12:31 is cited by the Bauer-Arndt-Gingrich-Danker lexicon as having a 'good sense', meaning 'strive, desire, exert one's self earnestly'. So the Hebrew and the Greek which the KJV translates in both places as 'covet' have completely different meanings.

The reader of English in 1611 or later years would probably understand the different shadings suggested by context. But language changes over time, and now 'covet' has lost much of the 'good sense' shading in our present day. Examining the original Greek and Hebrew resolves the apparent contradiction entirely.

Problem: One of the Ten Commandments says, “You shall not covet” (Ex. 20:17). Yet Paul encourages the Christian church at Corinth to “covet earnestly the best gifts” (1 Cor. 12:31,kjv).

Solution: “Covet” is used here in the good sense of “earnestly desire” (nkjv), rather than in the bad sense of an unlawful craving of what does not belong to us.