Return series even when duplicated (#414)

The engine returns an error when duplicate series are present in
the final result. It would still be useful to return the result
in addition to the error for debugging purposes.

Signed-off-by: Filip Petkovski <[email protected]>

Author: fpetkovski

engine/engine.go

@@ -451,10 +451,10 @@ loop:
 			matrix = append(matrix, s)
 		}
 		sort.Sort(matrix)
+		ret.Value = matrix
 		if matrix.ContainsSameLabelset() {
 			return newErrResult(ret, extlabels.ErrDuplicateLabelSet)
 		}
-		ret.Value = matrix
 		return ret
 	}