For example a debt collector came after me for someone else's debt because I had the surname, initials and lived in the same area as someone they could not locate. I had to get very assertive with them to tell them to leave me alone before they stopped telling me I was liable. Debt collectors don't care who pays, as long as someone does.
A lot of this is just down to laziness of the DMC. For them they just have a client name, and they'll likely just run a quick search on the electoral register and use those details to chase you down. Majority of the time it is correct, but it's the incorrect ones that takes them ages to figure out.
Case in point, we moved into a house towards the end of summer. Previous occupants didn't settle the energy bill. Energy company has sold the debt on, and I've now had 4/5 letters through the post asking about the debt. If the DMC did a little bit more research, they'd find out that the previous occupants no longer live there, and thus sending them payment demands is futile.