Every astronaut launching on a Soyuz has a seat custom-made (source). However, I don't believe astronauts typically return on their original Soyuz, and I know that they don't always. In this case, are seats swapped, or are the seats still generic enough that someone else's seat can be used for return?
When the crew are going to return on a different Soyuz, they swap the seat liners beforehand.
"One of the first tasks for the Soyuz commanders after arrival was to swap the astronaut’s seat-liners to their new ferry." - http://www.esa.int/Our_Activities/Human_Spaceflight/iriss/Spacecraft_swap