That depends on the listing agreement.....and the agents involved with the transaction.
What does the listing state in regard to what you, as the tenant, will have to pay?
It should be stipulated in advance.
Personally, I don't look for another commission if the lease is extended, nor do I expect 2 commissions if it is a 2 year lease..........it's expensive enough for a tenant to come up with the commission, the first month's rent and the security deposit all at once, in advance of moving in (in my area we get 1.5 month's rent as security).
I see rentals as more of an accommodation, as my business is really based on sales and listings, so I am happy to help get someone settled in a new home.
What the expectation is in your area may be entirely diffrent.