@CurlsnSunshinetime4tea "i have counted 3 instances where my dh family tree has married into my family tree (or vice versa) over the course of a few hundred years.
both of us have french canadian heritage"
French Canadians as a group are known for having intermarried quite a lot over the generations. Not only those in Canada but also Cajuns as well. (Cajuns are the descendants of Catholic French Canadians who were expelled from Canada and moved to present day Louisiana in the USA during the 1750s/60s)
But they weren't unique in this, this has also happened with other groups, for example early US colonial families, Puerto Ricans and Ashkenazi Jews.
"...it made using the ancestry site confusing when researching and seeing names highlighted as being related to me whilst working on dh tree."
Marrying within a limited range of families is referred to as "endogamy" and, yes, it does mean that you get these sorts of things happening.
I don't have any French Canadian connections but I do have connections with early US colonial families. When talking to distant relations, who are descendants of these families and are also studying their family tree and DNA, they have told me that they have found literally dozens of examples like this, finding ancestors on eg their father's tree who has a DNA link to their mother or that both their mother and their father have a DNA link to the same person.
Another thing with endogamy is that you end up with many more matches and also a lot of shared matches, precisely because families intermarried so much.
I have been told that it is not uncommon for French Canadians and Cajuns to have over 100,000 matches on Ancestry and it's a similar thing for those from early colonial US families to have 80-100,000+ matches.
In comparison, the branch of that family that I come from, stayed at home rather than moving to America and I come from a long and distinguished line of ... farm labourers and domestic servants. The men generally worked on farms and the unmarried women worked in domestic service or as laundresses.
There was the occasional butcher, baker and stocking maker though (but no candle stick makers!), a couple of blacksmiths and even one rabbit catcher but, on the whole, I come from a long line of what Americans (and maybe Canadians?) would probably call "rural rednecks" and almost all have lived within a 35 mile radius of a particular city in England (in fact, most lived within about a 20 mile radius) in small rural villages.
Despite this, there has been relatively little intermarrying between the different families which is probably down to the larger populations in England.
As a result, for example, my mum has around 20,000 matches on Ancestry, my dad has 18,000 and I have 16,000 compared with the much larger figures that people from endogamous communities come across.