A TCP connection (which underlies HTTP and many other protocols) is uniquely (at a given point in time) defined by 4 parameters:
- The local IP
- The local port
- The remote IP
- The remote port
Even if you make the same request twice simultaneously from the same computer, even with the two IP addresses identical and the destination port identical, the source port will be different.
Likewise, if you have two requests coming from two devices going through the same NAT device, the NAT device will use different source ports. Depending on the device, it may either keep the original source ports (and only change one if there’s a conflict), or always assign a new source port independently of the original source port.
The NAT device will then keep for each connection a mapping in its translation table which states that external connection (external IP, external source port, destination IP, destination port) is mapped to internal connection (internal host IP, internal host source port, destination IP, destination port).