The following code prints 2:

new MyEntity();
new MyEntity();
var distinctTypes = Query.All<MyEntity>()
    .Select(it => it.GetType())
    .Distinct()
    .Count();
Console.WriteLine(distinctTypes);

It executes sql query with distinct on Id and TypeId:

SELECT COUNT_BIG(*) AS [c01umn] FROM (SELECT DISTINCT [a].[Id], 100 AS [TypeId] FROM [dbo].[MyEntity] [a]) [b];