Collapser.cs

public class Collapser<TResult>
{
    private SemaphoreSlim semaphore = new SemaphoreSlim(1, 1);
    private long windowInTicks;
    private long nextRun;
    private TResult lastResult;

    public Collapser(TimeSpan window)
    {
        this.windowInTicks = window.Ticks;
    }

    public async Task<TResult> ExecuteAsync(Func<CancellationToken, Task<TResult>> innerAction, CancellationToken cancellationToken)
    {
        long requestStart = DateTime.UtcNow.Ticks;

        try
        {
            await semaphore.WaitAsync();

            if (requestStart <= nextRun)
            {
                return this.lastResult;
            }

            this.lastResult = await innerAction(cancellationToken);

            this.nextRun = requestStart + windowInTicks;
            return lastResult;
        }
        finally
        {
            semaphore.Release();
        }
    }
}