Hi Ian I think i might be able to help you a bit with Margaret Duffy.
I have a Margaret Duffy in my family tree, born about 1851. Her mother, Rosanna Duffy was born about 1826. Both would fit in with your 1871 census results, and a lot of my family lived around the Platt Lane area (an ancestor on my maternal grandma's side owned 95 houses in the area).
Im not so sure on Bridget Duffy. I do have a Bridget who is Margarets sister. She is one of a set of twins, Bridget & Mary, but I have them as being born about 1860 which puts them a little older than your Bridget.
If this is your Margaret and Rosanna Duffy, however, the details are below:
Patrick Duffy b. c1791 ---- married ---- Hannah (Unknown) b. c1790
The children I know of were: Mary c1816-1867; Bridget & Rosanna b. c1826 (twins); Patrick b. c1829
Rosanna married John Duffy (b. c1821) in 1845 at St Peters in Bolton, Lancashire but they moved back to wigan as they are on the 1851 census in Kerfoot Row, Wigan.
The children I know of were: Thomas b. c1846; Hannah b. c1848; Margaret b. c1851; Patrick b. c1856; Mary & Bridget (twins) b. c1860