Actually, there is no clear-cut line for which medals you should get in the same category.
On one hand, we have similar medals granted alongside each other:
- Explorer and Conqueror are achieved together if you complete all generated levels (
example which is mine, but there are plenty of others)
- My Chessmaster Token/Cross medals were achieved together if you meet the requirement for the Cross (the difference is between UV clear and N! clear,
example)
- Any medals that require no damage clear (such as Hell Arena Key) also grant the just-plain clear (such as Hell Champion Medal) along side it (
example)
On the other hand, some of the medals are exclusive:
- UAC Bronze/Silver/Gold cluster (obv)
- Compet-n Silver/Gold/Platinum (just check a speedrun, but here's an
example for Gold against an
example for Silver)
- Minor/Major Icarus Cross (see Compet ones for Minor, and this
example for Major)
- Untouchable Badge/Medal (
example for Badge,
example for Medal)
Frankly I would prefer if all medals were exclusive, because it's a lot easier to keep track of how many times you actually did something. (If I want to know all the non-no damage clears for HArena, for instance, I have to subtract off all my Keys. This would be doable if all medals were at least consistent for the entire set, but they're not.) The other way tends to bloat the amount of medals you've actually earned and it's kinda meh when you're counting them up. If there were "achievements" for getting so many medals it would make sense, but IMO we should get one medal per "category" and nothing more.
As for badges, since they're one-time deals in the first place, getting lesser badges is a convenient way to collect a lot at once (UV clear to get Bronze-Gold, for instance). Because there IS a reason to have so many badges (ranks, which unlock stuff for at least a while) this is a practical use of the method.
EDIT: FYI,
this has been "discussed" briefly before. It's probably a good idea to bring it up again, however, since there a lot of new badges and the inconsistency is a little frustrating.