I have been putting aside 2.5 % of my salary as zakat and giving it to the poor. Should I pay zakat again at the end of the year?

It is permissible to pay zakat in advance. If you give the poor a sum equal to or in excess of your annual zakat as you receive your salary, intending such payments as your zakat for that year, all you need to do at the end of the year is to make sure that what you have paid is equal to or in excess of the sum you are obligated to pay as zakat. On your zakat date, you should calculate how much you owe in zakat. If what you have paid is less than your due zakat, then you must pay the outstanding sum. But if it is more, then the excess is counted as sadaqah [voluntary alms] or charity.
Having said that, I should point out to you that if your income is all that you have, then you are paying more than you need to. Zakat is payable on any property you have that is likely to grow. Thus, it is payable on savings, investments, capital, profits etc. Your salary is not the only thing that is zakatable. If you do not have much of the other types of wealth, then what you have been doing is more than enough. Otherwise, you may need to add to what you pay monthly.
And God the Almighty knows best.