Q:

Proxy generation in WCF

In IDesign WCF coding standard it says :
"Do not duplicate proxy code. If two or more clients use the same contract, factor the proxy to a separate class library." 
I would like to know what is the advantage of the above state ?

A:

Apart from the general principle of DRY (don't repeat yourself), it avoids the potential confusion of having multiple types with the same names and members.
Of course if your two or more clients are totally independent (separate Visual Studio solutions), it's OK for each to have its own proxy.

