In the case of a Multiple-listing (or MLS), you would normally give a certain estate agent the sole and exclusive mandate to sell your property for a certain period, after which this agent, the "listing agent", lists the property with a computerised central service.
The information is then available to other participating agents who are free to sell your home. The selling agent and the listing agent are then in agreement that the agent's commission is shared between them. You do not pay more agents' commission. It remains the same amount agreed upon, but the agents share it between themselves.