the only problem with her being a silver duckwing, true duckwing roos (of any color) have a white triangle pattern on their wings. (also, true duckwings can be sexed as soon as they start feathering, as the breast will be salmon on pullets and black on cockerels. (and most can be sexed at hatch by the head coloration)
Silver grey dorkings are silver duckwing. (colored dorkings are gold duckwing)
a pic of a pair of 3 week old dorking (silver duckwing) chicks...
2 pics off feathersite showing the duckwing pattern (silver and gold)
now all that being said, she is a pretty girl, i have one just like her, but being a mixed breed one parent may have been duckwing, but too many other colors are dominant over this and override most of the pattern.
your girl's coloration is amost more like my dark brahma (bantams) than silver duckwing IMO...