First lets define a network domain as a collection of internet addresses that map to NwkID's
The TTN would be an example of a network domain and would be in the form of a registry managed by the TTN community of reserved internet addresses that map to NwkID's.
For example, it might be each country has it's own NwkID and at that location is a packet forwarder which contains it's own list of Lora network servers for it's region.
So the network domain is the set of all Servers registered to each NwkID location.
It is indeed entirely possible that multiple network domains overlap and receive data from Lora Motes as the Motes themselves are not aware of the gateways that are in their range and it's not possible for gateways to determine if a mote belongs to it's network domain.
Ultimately though, the definitive owner of a particular address is the holder of the matching encryption keys. Wether or not the packet gets to the owner of those encryption keys is another thing.
For example in Netherlands you could say there is a KPN an TTN network domain
It might be that these two domains get linked by sharing a NwkID between them to form a single domain but lets consider them separate. I am not a member of the LoRa Alliance ( yet ) but I don't think the intension is to create a global registry of network ID's rather for different network domains to form and manage data exchange between them.
So to come back on your point. It might well be the case that your packets are entering multiple domains at the same time but when you Join ( assuming OTAA ) your encryption keys will only match to one of those domains. When your network server allocates your device an address it will assign your mote the NwkID that the server belongs to as part of the allocated address.
It is indeed possible that this allocated address will identically match the allocated address of another overlapping network domain in which case both network servers will receive all traffic for both devices.
However, only one of the servers will have the valid encryption keys and hence the encryption keys are acting as the final data filter.
The NwkID's though remain a useful partitioning strategy to optimise network traffic through the network domain
So to summarise: Yes you can get duplicate addresses but the unique encryption keys are the final allocator of network traffic