Can I "duplicate" a stereo signal to 5 channels? Are there any impedance issues?
All those audio inputs have quite a high input impedance (>5K and probably higher... my Denon AVR specifies it at 47K), so you shouldn't expect any loading problems even if you parallel all them together, because your audio source with have probably &lt; 100 Ohm output impedance.

If you want to be on the safe side then add to your design a buffer between the audio source and the point where all the audio inputs are connected.