Suppose that I have one node that is associated with 2 AP that have the same BSSID (MAC address) and the two AP are transmitting data to the STA simultaneously (downlink), and they 1m far away each other (not hidden). Under such circumstances, each AP use CSMA/CA to reserve the channel and transmits data accordingly, it will be a problem for CSMA/CA mechanism if both share the MAC address? What problem do you expect under that circumstances? Considering that IEEE explicitly tells that one client can only associate with one AP only.
Actually, the physical carrier sensing part of the CSMA/CA considers only the energy detection mechanism to assess the medium. Any energy present in channel that exceeds the clear channel assessment threshold will infer that the medium is busy. This is done at the physical layer, without considering any MAC layer parameter (BSSID).
In general, duplicating a MAC address creates problems, no question about that.
In cabled Ethernets, this will be true whenever the duplicate MAC addresses exist within the same layer 2 structure. Meaning, if the these duplicated MAC addresses exist in separate IP subnets, and the separate IP subnets DO NOT share the same Ethernet broadcast domain, then all is well. But even in cases where the duplicated MAC address are in separate IP subnets which share the same Ethernet broadcast domain, you can expect problems.
WiFi is no different. It looks like an Ethernet broadcast domain, and therefore APs should not share the same MAC address. (Obviously, this isn't a real problem if the two APs are way beyond RF range of each other.)
Usually, coexisting APs use different frequencies (Channels) to communicate with STAs to avoid interference between nearby system. In that case it will be two different collision domains. Moreover, as per IEEE 802.11 standard "Att any given instant, a STA may be associated with no more than one AP"
In your case it is a single collision domain and theoritically the MAC address conflict problem should occur. But, as I can see it is only meant for downlink traffic the same MAC address would be present as source address, and in that case it should not be much of a problem.
It's true that each AP should be operating at a different frequency, if they are close together. But consider the case of an Ethernet catenet which includes two or more APs, operating purely at layer 2, to distribute wireless coverage in different locations. The APs should have unique MAC addresses, if for no other reason, to run STP or RSTP in the Ethernet that ties them together.
The general rule is that MAC addresses of two or more devices may be identical only if the the devices are operating over different IP subnets. The problem is that the manufacturer of the device doesn't necessarily know how the clients will be using their devices. So the safe bet is to have unique MAC addresses for each device (each device interface, to be precise).
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