Normally, agents charge 1 month equivalent of rental for a new lease agreement.

If the agreement is a 1 year plus 1 year duration, do they charge another 1 month upon extension to the 2nd year?

How about subsequent to that eg. 3rd year?

Or can owner DIY the agreement for 2nd year and onwards?

Agent only charge once. Renewal or extension only pay for stamp duty.

Technically since agent's are paid for their services of finding and securing tenants, they should be responsible if the tenant breaches the contract and ran away in the first year but sadly that's not the general practice in Malaysia.