I have one doubt after giving zakah. Last year I came to know that the person whom I gave zakah was not eligible for receiving the zakah. Should I have to pay zakah again?

Answered according to Hanafi Fiqh by Darulifta-Deoband.org

بسم الله الرحمن الرحيم

(Fatwa: 1024/1024/M=1434)

If you give him zakah considering him eligible to receive zakah then your zakah was paid. It is not necessary to pay it again.
دفع بتحر لمن ظنه مصرفاً …… إن بان غناه أو كونه ذمقياً أو أنه أبوه أو ابنه أو امراته أو هاشمي لا يعيد (درمختار)

Allah knows Best!

Darul Ifta,
Darul Uloom Deoband