what costs are involved in selling a duplex? do you have to charge a listing fee? are some charges negotiable?

You need to contact a local real estate agent. Fees and cost vary by location. Most Fees have some room for negations but realize this is not a sellers market... at least not in any place in the US that I am aware of at this time. If you attempt to cut back on commissions, you will probably reduce the agents ability to locate qualified buyers.

Many closing cost are determined by local custom and standard practice... these vary greatly depending on location; state and city, and while some are still negotiable you should be smart enough to understand that the more you push over to the buyer side.. the less attractive your property becomes.

Get what is typical and standard from your area REALTORŪ and understand it. Then ask your REALTORŪ for recommendations and the effects of any attempts to move cost and fees that are not typical for your area. They will effect your sale and the more shifts the higher the negative effect will be. Again this is NOT a sellers market anymore.