It is your right to negotiate the estate agent's commission, although please remember that this is an agent's income and when you want to pay as little as possible for a professional service, chances are that you will not receive a full service.
If you realise that two different estate agents have introduced the same potential buyer to your property, let the estate agents know about it immediately. If the buyer purchases your property through the second estate agent, you may be liable to pay double commission, because the first estate agent can claim the full commission amount as well.