It's generally advised never to re use water - resources that talk about water safety when injecting will often list this as the least preferable option. The major risk is contamination with blood borne viruses, but obviously this is a risk that comes from sharing ampoules, not reusing them alone.
The major thing you should be concerned about is that water that has been standing can grow a lot of bacteria really quickly - if the ampoule has been opened, it is no longer sterile and may have a dangerously high bacterial content. If you don't have a new ampoule, your best bet is to boil and cool water from the tap instead of reusing the ampoule.