There are some apparent exceptions to Duverger's Law, such as Canada, which has long had a multiparty system in spite of using a voting system similar to that of the US. However, a friend of mine who follows Canadian politics tells me that what really happens in Canada isn't that far from what you would expect given Duverger's Law. Currently, the three largest parties are the Conservative Party, the New Democratic Party (NDP), and the Liberal Party. It used to be that the NDP was a relatively small party with positions well to the left of the Liberals, but this is no longer true. Instead of offering Canadian voters two different flavors of liberalism, the current situation is that in any given election for any given seat in parliament, the NDP candidate and the Liberal candidate put a lot of effort into arguing over who has the best chance of beating the Conservatives.

Note that the Conservative Party is the outcome of a merger of two conservative parties (which were themselves split from a previous union). When the two parties were separate, neither could win, but after uniting they were able to take government (since the left was vote-splitting more than the right).