I may have found Duncan's arrival in Canada. There is a Duncan MacLean who was single, 23, a joiner and born in Argyle who was travelling on the Cassandra which departed from Glasgow and arrived in Montreal on 15 May 1907. It indicates he was destined for Winnipeg. Here's a link to the image (he's listed with other MacLeans but he has a separate ticket number so it looks like he was travelling alone):
http://data2.collectionscanada.gc.ca/e/e147/e003663502.jpgThe citizenship application for Duncan Rankin MacLean indicates he was born 7 Oct 1883 in Oban, Scotland. He was a brakeman for the Great Northern Railway. He arrived in the US about 16 May 1913. He crossed the border coming from Vancouver, BC. His application was made in Snohomish County, Washington.
I found a border crossing document for Duncan which indicates he arrived in Blaine, Washington. His occupation is listed as railway brakeman and his last permanent residence was Saskatoon, Saskatchewan. In the column for listing their closest relative from the country from which they came, Duncan lists his brother Robert of Winnipeg, Manitoba.
It looks like Duncan returned to Scotland for a visit. Duncan (43, trainman) and Margaret (32) were on the Tuscania which departed from New York and arrived in London on 21 Jun 1927. The passenger list indicates they were going to be staying at Argyll Park Terrace, Edinburgh. It also indicates they were US citizens but their last permanent residence was Canada.
It looks like they returned to Canada on the Cameronia which departed Glasgow and arrived in New York on 25 Aug 1927. Duncan is listed as 43, trainman, born Argyllshire. He and Margaret were returning to Vancouver. The passenger list indicated he had been living in Canada from 1923 to 1927 while Margaret had been here from 1920 to 1927. His closest relative in the country he left was his mother Margaret at the address listed on the incoming passenger list. It would appear that after Duncan got his US citizenship he met Margaret, married her in BC and then they remained there.