I recently had to use a spare donut since I got a flat tire. I was stranded on the road and my spare donut did not have much air. Someone helped me and changed the flat tire with the donut and filled it with 'fix a flat' can. Now, I have replaced the donut with a new tire. However, I am not sure if I can use the donut in the future (if ever needed) since it was inflated using fix a flat?? Someone mentioned, once you use fix-a flat, you cannot re-use the same tire.
You absolutely can use it again. The places that change tires try to discourage people from using fix-a-flat because it makes a mess when they go to remove a tire, but it doesn't take away from the integrity of the tire.
That being said, it's probably a good call to keep another can of fix-a-flat in the trunk. In my experience, fix-a-flat doesn't do a great job at fixing flat tires, though what it does do a good job of is inflating tires that have slow leaks. Since your spare was flat, it probably has a slow leak, and when you go to use it again, it might be flat. If that happens, you can just use the extra can of fix-a-flat to inflate it again.
Best of luck!