The homestead act wasn't passed until 1862. If your John bought land out west before then, it was for some other reason.
There are a number of federal land purchases in Indiana under the name "John Duncan" at the Bureau of Land Management website, but because it is a common name, you would need to narrow down where his land actually was in order to know if any of those transactions listed might be the one you're interested in.
Each listing at the BLM site has an accompanying file of information at NARA. The transactions that are marked: "cash entry" (when you click on them) have very little information, in fact, they usually just consist of a small dated cash receipt showing how much was paid, and also a slip showing the coordinates of the land that was purchased.
Any transactions listed as "homestead" or "scrip" or some other act of Congress that the land was sold under may have more information than cash-entry transactions. Homestead files are very informative.
The BLM site has the patent certificates viewable online, but anything else in the file would have to be obtained from NARA.
The BLM website where you can search for patents in federal land states is:www.glorecords.blm.gov
There is also a fine tutorial to the grid system at that site. Good luck in your search!