No, seals are not included in the legal terms of the agreement. In Pasal 1320 of the KUHPer (Civil Code), the terms of the agreement are valid if:
There is an agreement
The prowess of the parties
There’s a certain thing
The object of the agreement
So, if the agreement uses a stamp duty but does not meet the four conditions above, then the status is still invalid in the eyes of the law.