Sigma phase can sometimes be found in austenitic stainless steel weld metal.  It is most commonly encountered after long exposure times at temperatures of 600 to 870 degrees celcius.  Sigma phase is associated with ferrite (contained in duplex stainless steels) and thus presence of Cr & Mo will promote sigma phase formation.  The formation of sigma at these higher temperatures remains as a hard brittle segregate at ambient temperatures and its presence can be deleterious to the mechanical properties of the material, especially toughness.

VN

The presence of sigma in duplex is quite possible, but very unusual, since duplex stainless steels are not used at temperatures that would promote sigma.  If this has happened, then your corrosion resistance has suffered significantly also due to precipitation of other phases.