No, however if the cables from the PSU to the Duet are long or too thin then you could be getting excessive voltage drop on them. Similarly for the cable from the Duet to the bed heater. To check this, use a multimeter to measure the voltage at the PSU output terminals, the Duet VIN terminals, and at the bed heater.
If you are getting excessive voltage drop between the PSU and the Duet, you can use 2 sets of wires between the PSU and the Duet. At the Duet end of the cables, use 2-into-1 ferrules. At the PSU end, use 2 sets of output terminals.