I made a covenant with Allaah The Exalted while placing my hand on the Mus-haf (i.e. a copy of the Quran) that I will not commit a certain sin again, but later on I yielded to my weakness and committed it. I offered one expiation for breaking that oath by feeding ten needy people. However, when I swore that oath I repeated it many times so as to serve as a deterrent to committing that sin again and in order to render the expiation harder for me. I thought that I should offer expiation for each oath that I had sworn. Is the expiation that I have offered sufficient for me or should I offer expiation for each of those oaths because of my intention at that time?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

The context of your question apparently indicates that you should offer expiation for each oath that you swore, based on your intention when you swore them. Offering one expiation does not suffice you nor would it expiate all the broken oaths in this case. This is because the oath is interpreted according to the intention of the oath-taker, as Muslims scholars have stated. The number of expiations to be given depends on the wording of the oath and the intention of the oath-taker. Muslim scholars stated that it is obligatory on the person to offer expiation for violating his covenant to Allaah The Exalted. This is the opinion of Shaykh Al-Islam Ibn Taymiyyah .

The Maaliki scholar Ibn Abi Zayd said in his commentary on the book Ar-Risaalah, "Whoever says, 'Due on me by the Covenant of Allaah and His Promise (to do such-and such-),' while taking an oath and then breaks it should offer two expiations. However, when the oath-taker repeats his oath for the purpose of confirmation, he should offer only one expiation for breaking it. If he intends the second oath to be confirmatory of the first or had no particular intention when he made the second one, he is required to offer only one expiation should he break his oaths, according to the scholars' agreement. Nevertheless, if he intends by repeating the oath giving multiple expiations, he is obliged to expiate for all the repeated oaths according to the scholars' agreement." [Ar-Risaalah]

In your case, it is incumbent on you to offer expiation for each of the repeated oaths despite the fact that you had sworn them about the same thing due to your intention of making the matter harder for you.

 Allaah Knows best.