If I understand the loop and one instance's functionality, then the checkboxes for them should be exclusive. It should not be possible to "loop" and "one instance" the same task at once! If I understand the funcionality of those options worng please just correct me on this!
I checked the source. The "one instance" option only means that if the timer fires again, it will not start a new instance if the old instance is still running. If the task process, however, was finished, it will run a new instance. When disabling the "one instance" option, it will launch a new task process for each timeout, even if that would result in multiple instances running.