UWP Background Task - Application Trigger - Cannot create a file when that file already exists

In UWP I'm trying to create a simple background task on demand. So when I click a button, I want a background task to kick off and process some files in the background.

Below is what my basic Task code is:

public class BackgroundSyncService
{
    private ApplicationTrigger trigger = new ApplicationTrigger();

    public async Task Start()
    {
        if (IsRegistered())
            Deregister();

        BackgroundExecutionManager.RemoveAccess();

        // does this prompt everytime?
        await BackgroundExecutionManager.RequestAccessAsync();

        var builder = new BackgroundTaskBuilder();

        builder.Name = "BackgroundTask";
        builder.TaskEntryPoint = typeof(BackgroundTask).FullName;

        builder.SetTrigger(trigger);
        BackgroundTaskRegistration task = builder.Register();

        task.Completed += Task_Completed;

        var result = await trigger.RequestAsync();
    }

    private void Task_Completed(BackgroundTaskRegistration sender, BackgroundTaskCompletedEventArgs args)
    {
        // alert UI
    }

    public void Stop()
    {
        Deregister();
    }

    private void Deregister()
    {
        var taskName = "BackgroundTask";

        foreach (var task in BackgroundTaskRegistration.AllTasks)
            if (task.Value.Name == taskName)
                task.Value.Unregister(true);
    }

    private bool IsRegistered()
    {
        var taskName = "BackgroundTask";

        foreach (var task in BackgroundTaskRegistration.AllTasks)
            if (task.Value.Name == taskName)
                return true;

        return false;
    }
}

When the code hits builder.Register() it throws the following exception:

System.Exception: 'Cannot create a file when that file already exists. (Exception from HRESULT: 0x800700B7)'

Not sure what I'm doing wrong here or what this error means in context of a Background Task?