I've got potions from levels 1 through 9. When you use 9, you're supposed to get 8, etc. What actually happens is unpredictable. I use 9, I get 5 or 4, I use that, I get 3, then it goes away entirely.
I imagine it works the same way in reverse - picking up another 1 doesn't increase the level 1, but I'm not sure.