CollapserTest.cs

[TestMethod]
public async Task Collapser_WithMultipleExecutions_OnlyRunsOnce()
{
    int i = 0;
    var collapser = new Collapser<int>(TimeSpan.FromMilliseconds(100));
    Func<CancellationToken, Task<int>> action =  (ct) => { i++; return Task.FromResult(i); };

    await collapser.ExecuteAsync(action, default);
    await collapser.ExecuteAsync(action, default);
    await collapser.ExecuteAsync(action, default);
    var result = await collapser.ExecuteAsync(action, default);

    i.Should().Be(1);
}