SIN-SFO arrives at 9AM. Let's say you are booked SFO-LAS 11AM on the same ticket and intend to no show.

Let's say you really need to fly SFO-SBA instead (a route where only UA offers nonstop service).
Let's say you further want to buy a discounted SFO-SBA fare in advance.

If you book SFO-SBA for ~11AM, you are at high risk for a UA's automated duplicate booking process producing unwanted cancellations.
If you book SFO-SBA for ~4PM, you are at medium risk.
If you book SFO-SBA for -10PM, you are at low risk.
If you book SFO-SBA for ~11AM next day, you are at no risk.

If you really want the SFO-SBA for ~11AM same-day, you'll need to mitigate risk. One option is to get SFO-LAS ~11AM flight changed to much later in the day or early the next morning. Another option is to book SFO-SBA with a different name (JOHNQUINCY ADAMS, a birthday typo).