Two of these examples were discussed in an American Political Science Review* article back in 1982. Canada and India are well-known counter-examples to Duverger's law.
In an examination of 121 elections of which 30 were conducted under plurality rules ("winner take all"), 7 had significant numbers of votes for a third party. All 7 of those were in Canada. The researcher (Rae's) conclusion was that Canada's decentralized government provides incentives for parties to also be decentralized (because provincial control is enough of a reward). In Canada, this led to regionalization of elections: a party might be one of the Big 2 in one province, but considered a third-party in another. In some sense, this preserves Duverger's law at the sub-national level. The rules of the election in each region lead to the expected conclusion, but at the national level don't appear to follow Duverger's Law at all.
India is harder to explain. At the time of the article, there were two explanations:
- First, many of the "parties" are a form of identity-politics. Rather than a serious strategic organization which is attempting to win elections, they provide a kind of emotional satisfaction to members. Since these parties aren't strategic they aren't expected to have any meaningful impact on the election, and the electoral rules are capable of supporting multiple parties.
- Second, there is a spatial voting explanation. At the time of writing Congress was the dominant party in India, and it was believed to occupy the policy-space which attracts the median voter (and thus, it is the optimal position to occupy). The rest of the parties were struggling to pick up marginal votes in smaller policy-spaces. Under this view, India is really a one-party system with a large number of minority parties. Duverger's law is violated by the existence of a single party which monolithically occupies the ideal policy-space.
The article proposes an alternative formulation of Duverger's law, which at least when I was in graduate school (early 2010's) was familiar:
Plurality election rules create and maintain a competitive two-party system except where (1) one party is almost always the Condorcet winner or (2) elections are contested regionally, rather than nationally (paraphrased)
APSR is the most prestigious publication in political science, except in some specific niches which have their own publication.