re #8970 Check ws belongs to group before inserting it

This get rids of warning message in SANS reduction due to inserting
workspace to a group which already had that workspace.

Code/Mantid/scripts/SANS/isis_reduction_steps.py

@@ -430,8 +430,9 @@ def _keep_partial_results(self, sample_name, can_name):
         gp_name = 'sample_can_reductions'
         if mtd.doesExist(gp_name):
             gpr = mtd[gp_name]
-            gpr.add(sample_name)
-            gpr.add(can_name)
+            for wsname in [sample_name, can_name]:
+                if not gpr.contains(wsname):
+                    gpr.add(wsname)
         else:
             GroupWorkspaces([sample_name, can_name], OutputWorkspace=gp_name)