How to certify a document in usa (apart from notarization)

You have not given the nature of the agreement. Unless it is known, we shall not be knowning whether the agreement attracts stemp duty. Notarising or Indian Consulate signing is merely an attenstation and confirmation that the signatories are identified and signed in their presence and beyond that the notarising or Consulate signature does not carry any further value.

Therefore, it depends upon the nature of the agreement. If it attracts stamp duty, it has to be stamped as the rules and regulations applicable in the place it is executed and when it is received in India, it has to be examined what was the stamp duty actually paid and what is the stamp duty where the agreement needs to be executed. Diffrential if any has to be paid.

Therefore, the key is the nature of agreement and purpose of the agreement.

Yes, what is the necessity of attesting the document than notarising it?

As my colleagues suggested it depends up on the facts and circumstances of the agreements/ documents. But in general if the document is executed befoe the Notary Public and then legalised by the US Embasssy and then attested by the Ministry of Foreign Affiares, it would be acceptable to courts.