Same class name in multiple namespaces in Visual Studio

I'd just write a unit test that uses reflection. Something like:

var duplicateGroups = typeof(SomeTypeInAssembly).GetTypes()
    .GroupBy(t => t.Name)
    .Where(g => g.Count() != 1);
// Assert that duplicateGroups is empty

Exactly how you assert that the collection is empty will depend on your test framework - you'll also want to make sure that when the test does fail, it gives you a useful failure message, ideally a list of the types in each group.