Erasing and Reprogramming old EPROMS
I don't see any reason not to double program them (I don't think it can hurt much, and it could well help), unless your programmer is &quot;smart&quot; and refuses to start if the 'blank check' fails.
Pro programmers used variation in the Vdd to ensure full programming, if memory (ha) serves. The programmer programs at one Vdd and subsequently verifies at another to ensure there is a certain amount of margin.
Unless the chips have been used in a (horrific flashbacks) development environment where the poor engineers didn't have an emulator they will likely have been erased once at most, even if they are 20 or 30  years old. It's likely that erasing (high intensity UV) and programming (high voltages to cause tunneling) that ages them.