“a thing devoted to destruction.” If a person stole something that had been “devoted” for destruction, then that person became devoted to destruction as well. This is what happened to Achan and his family (Josh. 7:1-26; cp. Josh. 6:18). [For more on things “devoted” to Yahweh and devoted to destruction, see commentary on Josh. 6:17].
“detest, yes, detest it...abhor, yes, abhor.” Deuteronomy 7:26 uses a double polyptoton for emphasis. The figure of speech polyptoton is when a word occurs in two different forms set against each other for emphasis. In this case, the Hebrew words translated “detest” and “abhor” each occur first as an infinite and then as an imperative verb, thus the Hebrew more literally reads something such as, “you are to detest, detest! You are to abhor, abhor! [For the English translation of doubling the verb with “yes” between them, see commentary on Genesis 2:16].