Has your property got more meterage than the other homes on your development? i.e extra garden space, basement internal meterage, larger parking space and store room, all? What has been declared in your deeds and more importantly in the land registery? Have you requested a Nota simple of your property? It costs little and may be worth doing for your apartment and with a neighbours apartment too in order to compare. It is public knowledge so you do not need permission to request a registry note for a neighbor. http://www.registradores.org/principal/indexx.jsp All you require is their full name & address and if requesting online you can get a reply within 24 hours.
Ask your neighbor if they can provide you with a copy of their recent IBI bill too so that you can add it to your complaint documents.
IBI (impuesto de bienes inmuebles) is calculated in relation to the meterage of the plot, the qualification of the land (urban, rustic etc) and the registered land value (published land value tables in the area A.K.A Valores catastrales) which can vary between one development to the other. If your neighbor bought their property a year or two before you on the earlier phase of your devlopment you may find that the IBI varies however 60% is a little worrying however not impossible.
If the Patronato do not provide you with a reply that is satisfying enough for you the best thing to do is consult a lawyer. They can inform you of the best measures to take. Even if you prefer to do all the ground work yourself a lawyer can at least help point you in the right direction and aid you with your claim.
Do not default on payment, until it is proven that there has been a mistake you are legally obliged to pay the IBI and they are quick at acting on non payment of council rates…
Good luck and hope that the matter is resolved soon.