Recently I performed Aqeeqah on the 6th day, for my son, on the understanding that it has be performed within first 7 days of the birth of the child. Please advise whether this is acceptable or not in the light of Hadith, and whether I have to repeat the same.

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is a disagreement among Muslim scholars over this issue. But we believe that the Aqiqa you have done for your baby suffice him and you do not have to do it again. There are two reasons for this opinion.
First the Shafi and Hanbali schools of Fiqh state that the Aqiqa suffice even if slaughtered before the seventh day of the newborn child.
Second, you are excused because you did what you did believing that it is the right thing and ignorance can be taken as an excuse in some matters like this one.
For more details please refer to fatwa no.: 81462 and 81275.
Allah knows best.