If PCs short rest more than once a day, do they spend hit dice both times at level 1? Or do they have to long rest in order to get their hit dice back?
Per the rules on resting:
When you take a short rest, you may spend hit dice in order to regain hit points.
A 1st level character will only have 1 hit die, so that's all they can spend - you would roll that die and add your Constitution modifier. Once it's spent, you cannot spend it again until you get it back, which leads us to...
You only get hit dice back by taking a long rest. When you do so you get back half your normal amount of hit dice, but a minimum of 1 (so a 1st level character would get back their one die).