[megablast]

Every single airline does this. In part, because people would often book returns because they were cheaper than one-ways.

[dingaling]

The majority of seats sold in Europe are now one-way / singles, due to the rise of low-cost carriers which don't offer anything else.

However it's still a good idea to book each individual flight in a different transaction. Avoids any shennanigans on the airline's part for no-shows and there's no downside for the passenger other than a slightly longer booking process.