I picked one out at random to have a look, and I'm not sure if this was just pure chance or not. One of the reported double-ups was on this map:
https://osu.ppy.sh/b/84785 username Kochiya Sanae. It turns out with a closer look that there are in fact two different players that somehow have the same name: a Korean player (
https://osu.ppy.sh/u/1183624) and a Chinese player (
https://osu.ppy.sh/u/395591). I used a hex editor to check both names and they appear to not have any special characters hidden in them to make them different (
http://puu.sh/9YZ6P/a6e0d4ad92.png), even when the encoding is set to UTF-8. I'll look at some others to see if they are similar cases.
Edit: It appears that the Korean player's page is accessed by having two spaces in the username when entering the username in the URL, but is only displayed as having one space by the website. (
https://osu.ppy.sh/u/Kochiya%20%20Sanae for the Korean player as opposed to
https://osu.ppy.sh/u/Kochiya%20Sanae for the Chinese player). I suspect that multiple spaces are being displayed as only one space on the website and ingame but are being stored internally as multiples.
Edit2: A similar cause checks out for the other osu!standard map in the list,
https://osu.ppy.sh/b/26065, username 2NE1 GOD. There are actually two different users: "2NE1 GOD" and "2ne1 GOD" (
https://osu.ppy.sh/u/1286261 and
https://osu.ppy.sh/u/1203723). I wonder how likely it is that they are multi-accounts.
Edit3: This doesn't seem to be the cause for the other game modes.
Edit4 (hopefully final edit):
Edit5: The other osu!standard double-up is also caused by a double space. The non-capitalised one has two spaces.
(found while scrapping scores from api)
