What is the required expiation for someone who swore not to do something specific, then did it and repented, then returned to it again, and does feeding 10 poor people count as expiation for a covenant with Allah?

Whoever covenants with Allah to obey Him or to abandon a sin, it is obligatory upon them to fulfill it. This is considered a vow if it is uttered verbally. However, if it is merely a thought or an utterance that does not convey a sense of commitment, then nothing is binding upon them. Whoever breaks their covenant must offer an expiation for an oath. The phrasing mentioned in the question does not fall under the ruling of a vow.