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Should we assume that Ann's maiden surname was also Mackenzie?
What other information do you have about Duncan and Isabella? Did they die in Scotland? What were the names of their children, in order?
From the indexes at
www.scotlandspeople.gov.uk I see that Isobel, daughter of William Mackenzie and Anne Mackenzie, was baptised on 4 February 1802 in the parish of Lochbroom.
There are several other baptisms of children with the same parents' names
Kenneth, 1808, parish of Urray
Isabel, 1810, parish of Urray
Ann, 1812, parish of Urray
John, 1816, parish of Urray
Roderick, 1833, parish of Gairloch
Anne, 1834, parish of Gairloch
However there are nine marriages of William M(a)ckenzies and Ann(e) M(a)ckenzies between 1784 and 1846, so these are almost certainly not the same couple.
From a transcription, there's a family in Contin in the 1851 census consisting of Duncan Beaton, 60, herd; wife Isabella, 50; and brother Donald, 90, pauper formerly labourer. Are these your Donald and Isabella?
In 1881, in Contin, are Rebecca McDonald, 65, shepherd's widow; sister Isabella Beaton, 80, shepherd's wife; and Duncan Beaton, brother-in-law, 83, formerly shepherd. Are they yours?
Isabella Mackenzie or Beaton, mother's maiden surname Mackenzie, died in Stranraer in 1888 aged 85. You can download a copy of her death certificate at modest cost from
www.scotlandspeople.gov.uk