Hi Gillian , decided to concentrate on John Duff to see what I could find , well at least another marriage, below is in date order :
23/4/1845 (Possible) Christening St Georges Douglas , F William, M Catharine Fell (IGI)
1851 Census Douglas Aged 2 (Only one possible) not with any of the other 16 Duffs (Lawsons Census Index)
1861 Census, Douglas aged 12 , servant , not with other Duffs (IGI)
1/6/1869 Marriage Braddan Mary Ann Boyne, both single J's F Robert (IGI)
5/6/1870 Christening Daughter Maude Amy , St Barnabas, Douglas (IGI)
1871 Census Douglas, Aged 23 , wife Mary Ann 21 , Maude A 10 mths. (Lawsons Census)
22/2/1872 Christening Daug. Edith Annie, St Barnabas, Braddan (?) (IGI)
24/2/1872 Ch Daug Elizabeth , Douglas (Same child ??) (IGI)
3/7/1873 Burial Wife Mary Ann , aged 23, Braddan (Lawsons)
16/2/1874 Marriage Douglas, Mary Cull (or Call/Coll) , John's father Robert
19/5/1874 Burial Daug Maude Mary(?), Braddan aged 5 (Lawsons)
25/10/1874 Birth Daug Mary Ann , Douglas (IGI)
20/5/1876 Ch Daug Margaret , Douglas , mother shown as Cullen (IGI)
1876-78 Death of wife Mary ?? No Listing anywhere.
20/10/1878 Marriage in Douglas to Margaret Campbell (Lawsons)
20/10/1878 John is posted as John Cuff , Widower , Margaret is 18 yrs , John's F is Robert.
6/9/1879 Ch Douglas , son John James (IGI)
24/11/1880 Birth Douglas, son William Robert (IGI)
1881 census Douglas AGed 29 , wife Margaret 21(Born Ireland) Elizabeth 8, Mary 6, Maggie 3, John J 2, William R 1 (IGI)
28/1/1883 Birth , Douglas, son Henry (IGI)
31/8/1886 Ch Douglas, Son Edward (IGI)
1/9/1886 death Douglas , son Edward , (IGI)
2/9/1887 burial Braddan , son Edward aged 6 days (one of these two entrys are wrong) (Lawsons)
1886-1891 ? Death of wife Margaret - could find no proof.
1891 Census , Douglas, Aged 42, Mary 15, Maggie 14, JJ 12, Robert 9, Henry 7 (No Elizabeth) (Lawsons Census)
2/6/1891 Marriage Daug Elizabeth to Robert Alfred Alexander, Douglas (IGI)
15/8/1893 Marriage of Daughter Mary Ann to Richard Stephenson , Douglas (IGI)
7/1/1895 Marriage of Daug Margaret to John Boulter , Douglas , (IGI)
1908 Approx. Marriage of son JJ to Mary Agnes Killen , unknown where IOM
20/1/1909 ch of Grandson Francis John Duff , Douglas , (IGI)
4/2/1912 Marriage of widowed Daug Margaret Boulter to Henry Cowin , Braddan, (IGI)
24/7/1926 Marriage to Annie Wilson , Braddan (IGI)

The 1911 Census has John James, Mary Agnes & Francis John living in Barrow in Furness (IGI)

Hope you can get something from this. I am able to read all the postings.

Cheers
David