Description of problem:
In FRR is not possible (as far as I know) to have the same neighbor defined twice (or more) with different ASN. As an example, if you try:
  neighbor 192.168.216.10 remote-as 65000
  neighbor 192.168.216.10 remote-as 65001
only one of the entries will get applied (and shown if running show running-config), not both.

So, MetalLB should not allow to create two BGPPeer objects with the same peerAddress.
In case this is a limitation specific to FRR (and not applicable to go-bgp), upon trying to create the secondary bgpPeer object the user should receive some kind of error like the following:
The BGPPeer "peer-65002" is invalid: spec.PeerAddress: Invalid value: 0xfdeb: Multiple equal peerAddress definitions not supported in FRR mode, ...

Version-Release number of selected component (if applicable):
Client Version: 4.10.0-0.nightly-2022-01-10-101431
Server Version: 4.10.0-0.nightly-2022-01-10-101431
metallb-operator.4.10.0-202201180823

How reproducible:
100%

Steps to Reproduce:
1. Create the first BGPPeer object:
    apiVersion: metallb.io/v1beta1
    kind: BGPPeer
    metadata:
      name: peer-65001
      namespace: metallb-system
    spec:
      peerAddress: 192.168.216.1
      peerASN: 65001
      myASN: 65000
2. Create the second BGPPeer object:
    apiVersion: metallb.io/v1beta1
    kind: BGPPeer
    metadata:
      name: peer-65002
      namespace: metallb-system
    spec:
      peerAddress: 192.168.216.1
      peerASN: 65002
      myASN: 65000

Actual results:
 - Both BGPPeers are created, however only one of them is correctly applied in the speaker configuration

Expected results:
 - The second BGPPeer creation should fail. In case this is a limitation specific to FRR (and not applicable to go-bgp) the user should receive some kind of error like the following:
The BGPPeer "peer-65002" is invalid: spec.PeerAddress: Invalid value: 0xfdeb: Multiple equal peerAddress definitions not supported in FRR mode, ...