What is stamp duty on a rent agreement, and who pays it?
A state tax on the agreement itself, calculated from rent, deposit and term — usually paid by the tenant, though the agreement decides.
Stamp duty makes the agreement a properly stamped instrument, which is what lets it be relied on as evidence. Each state sets its own rate and its own method — some charge a percentage of the total rent over the term, some add a component based on the deposit, and several apply a minimum or a cap.
Who pays is a matter of agreement rather than law in most cases. By convention the tenant pays the stamp duty and registration cost in much of India, but a well-drafted agreement states this explicitly instead of leaving it to custom.
Most states now allow e-stamping rather than physical stamp paper, which is faster and easier to verify. Because the rates and the method differ so much between states, get the figure for your state rather than applying one you have seen elsewhere.
General information about renting in India, not legal advice. Rules differ by state — check with your local Sub-Registrar office or a lawyer before acting on this.