Turning down an award wouldn’t erase the record of an injury, and it was the injury that mattered. They wouldn’t let a guy who lost an arm back into combat just because he turned down the Purple Heart, and the same goes for a lesser injury that still would bar one from combat.
A lot of injuries got you either reassigned or sent home, and it had nothing to do with either the fact that you’d “done enough,” or that anyone was playing the odds-- the next bullet you took had no way of knowing anything about what any previous bullets had done.
It had to do with whether it was cost effective to keep you on full salary while you rehabbed.
The length of your rehab, whether you’d be able to return to full duty, any duty you could perform while rehabbing, and what prior civilian skills you possessed that might make it easy to reassign you, or what specifically your job had been, and whether that would make it easy to reassign you, all played a role. It was easier to reassign a front-line medic with a slight limp-- lots of positions at hospitals-- than an infantryman.
Statistically, it was unlikely for someone to earn more than three Purple Hearts, but it was not impossible. Working his way up to Staff Sergeant, a guy named Albert Ireland earned nine Purple Hearts between 1941 and 1953 as a marine, serving in both WWII and Korea. He lived to tell about it, too, albeit, was finally medically discharged after the injury that got him the 9th award proved too much to allow him to remain in service.
SSGT Ireland has the record, so far, but he may not hold it forever, as better medicine, and way better rehab are allowing soldiers to return to combat after much graver wounds than ever before-- if they choose to. The current military policies are still being hammered out, but when and whether to allow willing soldiers back after injuries that would have earned a discharge in a previous generation are a real snarl. The idea that taking a huge hit has “earned” someone a discharge-- regardless of the eventual outcome vis a vis convalescence and rehab-- still persists, even though it has never been official policy, and the current military is all-volunteer, with more people than ever willing, and wanting to go back after recovering.