Bent exhaust valve
 

Recently while riding, my 1994 BMWR1100RS experience a loss in power and
had to slow down 'limping' to the workshop. After thorough checking, it was
found that one of the exhaust valve bent. New valve and spring were replaced
and the bike runs normal. I wonder would replacing the damaged valve but
retaining the other might caused the similar defect on the other valve in future?
Any advise is much appreciated.

Probably not. Bending a valve while riding is usually from over revving the engine. The valve train is designed to rev to a certain level. The other thing that is possible is the springs starting to get weak but this is less likely.