How do I stop people lending each other their cards?

What you fit: compulsory reads on the way out as well, and anti-passback, so the same credential cannot enter twice without having left; a speed gate or a corridor that only lets one person through at a time; the photo of whoever is passing on the control room screen; the image from that door's camera saved alongside the event; and a second factor on the doors that deserve it, because a PIN or a fingerprint does not get lent out with the card.

And the part that is not technical, which is usually the cause: if people lend each other cards it is almost always because the system gets in their way. Somebody needs to get into a place at six in the morning and does not have permission, so they ask for the card of someone who does. Before chasing the lending, it is worth reviewing the permissions, because if the reason is still there, the habit comes back.