Yes, i know people like this as well. I have no idea how you can separate the ones playing the system from those that are genuinely sick. I suspect you just can't.
I have had to sack a couple of people over the years who were 100% just taking the piss. They'd tell other staff they weren't really sick and then word would get back to me or other management. People really are daft, if you're faking illness at least don't tell your work colleagues about it!