This is the first time I'm installing Ipcops and the documenattion says that the network cards must be different.
Sure, got this out of the IPCop v1.4.0 Installation Manual: 1.2.1.5. Network Interfaces Your firewall will need at least 1 Ethernet cable and network interface card (NIC). It may need up to 4 NICs, depending on the network configuration you choose and your connection to the Internet. All NICs must be different physical cards (or their equivalent if you have multport cards).
You misunderstood this: you can have network cards of the same make and model; this quote says that you cannot use one card for more than one purpose. For example, if you have a green and an orange network, then you need one network card for the green network and one network card for the orange network; you cannot use one for both networks.
My ipcop does not want to work with 2 identical nics, it says auto detect failed when trying to add it, and also module already loaded when trying to add it manually. I wanted to add a 3rd interface but it seems impossible