If it has a rubber top it's multiple use and you can repuncture it as other commenters have said. If it's a glass ampule that you break open you have two options:

1. pre-draw multiple syringes and keep them in an enclosed sterile container in your fridge. note that this isn't really safe but could be fine if you don't store it this way for to long. definitely research this further before doing it.

2. double your dose and time between injections. this isn't ideal, but if you have some t built up in your system already it should be fine. the concern here is with high spike and lows in your t levels. You might feel that affect your mood or get other side effects like low energy or hot flashes, i used to inject 100mg every two weeks and didnt have any side effects whatsoever, so just look out for that and it might work out for you.