I see.
I responded to your other thread on pay issue. Wow, you have been subbing for 13 years? I really wish you had not taken the second job for that day since you already got a full pay that day. If you didn't want to decline, just end the call; the system wouldn't take it as either accept or decline. I'd assume that it'd be considered a call drop though I do not know what affect answering the call and listening to the job description would have if one doesn't accept it.
May be, once you find out that it's for the same day, you can decline and choose the reason as working in another district. I suggest this because it could have happened that way. Or you could have made a doctor's appointment already and was no longer available for the remainnig portion of that day and can choose "Not available". Thanks for sharing that experience. Something to watch out for.