This person connects to my network via WLAN, leaves then.
Then the person comes back and connects again.
How can the laptop be distinguished from others?
These three lines let me think you want to find an element that would make an host recognizable over the single session. I mean, the host connects/disconnects/re-connects and you still want to say that the last host connected is actually the host connected previously... Am i right?
If this is the case... well, as said by weVeg
, the three elements that identify a host on a network are baically hostname, IP addr, MAC addr. But all these three attributes can be changed/spoofed...
So if the host associates to the network, then disconnects, changes the three parameters we mentioned, and then reconnects back, i think you will never be able to establish if it's a new host or the host that disconnected few minutes before.
Obviously if you can conduct a deep scan on the host the first and the second time, and you can find some well recognizable services both the times, depending on the "normal" activity of your lan (you know how many people connect every day to your network), you can guess if it's likely or not that the two hosts connected in different times are actually the same host. But this is just a probability issue, you can never say that for sure.
Also there might be reserved IP, MAC filters, hostname filters, account based association (EAP/RADIUS) and so on applied to the network, and this would change something, but in every case you'll never know who a host is untill you see it with your eyes
Hope this was useful.