How we cite our quotes: (Section.Paragraph)
Quote #1
An act for granting and applying certain stamp duties, and other duties, in the British colonies and plantations in America, towards further defraying the expences [sic] of defending, protecting, and securing the same; and for amending such parts of the several acts of parliament relating to the trade and revenues of the said colonies and plantations, as direct the manner of determining and recovering the penalties and forfeitures therein mentioned. (1.1)
The D-word is invoked here. They're talking specifically about the amounts of money that will have to be paid for the stamps, although it would also be the duty of the colonists to pay them. In the crown's defense, it also stipulates exactly what this money is to be used for, and in fact it's something that's already been done.
Quote #2
[…] we, your Majesty's most dutiful and loyal subjects, the commons of Great Britain in parliament assembled. (1.2)
Parliament characterizes itself as the king's most dutiful subjects. Clearly, the idea of adhering to duty was valuable enough that it was used as a bit of a brag.
Quote #3
For every skin or piece of vellum or parchment, or sheet or piece of paper, on which shall be ingrossed [sic], written or printed, any declaration, plea, replication, rejoinder, demurrer, or other pleading, or any copy thereof, in any court of law within the British colonies and plantations in America, a stamp duty of three pence. (1.3)
This is a standard example of what you'd find in the first part of the law. You're going to see "duty" once a paragraph there. It's talking about how much that specific thing will cost to have a stamped version. And once again, you can see how the two slightly different definitions of "duty" relate to one another.