Amendment
Rule: If multiple key modes are present in a single beatmap set, the key mode must be denoted in the difficulty name.
Move to Guideline: If multiple key modes are present in a single beatmap set, you must have different names for all difficulties. osu! will overwrite difficulties of the same name.
[1]For aesthetic purposes, it's recommended to have clear and distinct spread names if you choose to avoid key naming for multiple spreads.
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Introduction
I'm assuming this rule is implemented to avoid any difficulties being overwritten, maybe also because the old website didn't support Key Amount display, and would instead only display Circle Size.
As discussed by MEGAtive (in [Proposal] osu!mania Ranking Criteria (whole draft) Post 52):
Adding here that you can't have exact same difficulty name because the filename will conflict with each other in the folder.
Player-side
There are no benefits and drawbacks, it's pretty clear on what keycount the map is from the beatmap information site.
Mapper-side
benefits
No more weird difficulty names that tries to add in key numbers.
This doesn't mean that they cannot add in the keycount in the difficulty, it's just optional.
Ranking-side
BNs to note to not enforce, but to advice to avoid complicated names if the difficulty naming is too complicated and also emphasize how two difficulty names cannot be the same due to file naming limitations.
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Justification
There are already multitude of ways to find key count, on the website, and in client.
This should not be enforced, instead, recommended due to file naming limitations.
Those who are seasoned in mapping would already have understood this limitation but would like to name difficulties on different keys differently, this will allow them to do so.
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Edits
17/6/2019 - Amendment to Guideline w.r.t. KeppSado's advice on aesthetic purposes (Ref: [1])