I wrote a pretty sloppy tool to go through recent contests. It comes to my attention that the following ids might be the same person:

dystopiawave
AestheticXJTU
lichunzhu
ChinaJapanFriendshipHospital
labyrinthulomycetes
EastFront
Zuikaku

For example, Q2 and Q3 in the latest contest:
dystopiawave as in https://leetcode.com/contest/weekly-contest-194/ranking/15/
AestheticXJTU in https://leetcode.com/contest/weekly-contest-194/ranking/57/

The full list as follows:

Weekly Contest 194:
dystopiawave (360) 2 3
AestheticXJTU (1401) 2 3

Biweekly Contest 28:
dystopiawave (298) 2
AestheticXJTU (5496) 2

Weekly Contest 185:
dystopiawave (278) 2
lichunzhu (415) 2

Biweekly Contest 23:
ChinaJapanFriendshipHospital (258) 4
labyrinthulomycetes (2226) 4

Weekly Contest 179:
ChinaJapanFriendshipHospital (180) 1 2 3 4
lichunzhu (1168) 1 3
AestheticXJTU (1649) 2 4

Weekly Contest 178:
labyrinthulomycetes (142) 1 2 3 4
lichunzhu (236) 1 2 3
AestheticXJTU (1849) 4

Weekly Contest 177:
AestheticXJTU (14) 1 2 3 4
labyrinthulomycetes (343) 3 4
lichunzhu (696) 1 2

Weekly Contest 176:
AestheticXJTU (66) 2
lichunzhu (140) 2

Weekly Contest 174:
lichunzhu (141) 1 2
AestheticXJTU (294) 1 2

Weekly Contest 173:
lichunzhu (231) 1 2 3 4
AestheticXJTU (419) 1 2 3 4

Weekly Contest 172:
lichunzhu (361) 1 2 3 4
AestheticXJTU (953) 1 2 3 4

Weekly Contest 170:
EastFront (25) 1 2 3 4
AestheticXJTU (280) 3 4
lichunzhu (346) 1 2

Weekly Contest 169:
EastFront (202) 1 2 3
AestheticXJTU (2640) 1 2 3

Biweekly Contest 16:
EastFront (25) 2 3 4
AestheticXJTU (103) 2 3 4

Weekly Contest 168:
EastFront (514) 1 2 3 4
Zuikaku (535) 1 2 3 4
AestheticXJTU (2512) 2

Weekly Contest 167:
Zuikaku (190) 1 2 3 4
EastFront (378) 1 2 3 4
AestheticXJTU (488) 1 2 3 4

Biweekly Contest 15:
Zuikaku (30) 1 2 3 4
lichunzhu (125) 1 3
EastFront (239) 2 4

Weekly Contest 166:
EastFront (265) 1 2 3 4
Zuikaku (289) 1 2 3 4
AestheticXJTU (747) 1 3 4

Weekly Contest 165:
Zuikaku (143) 1 2 3 4
lichunzhu (344) 1 2 3
AestheticXJTU (555) 4
EastFront (639) 4

Weekly Contest 165:
AestheticXJTU (555) 1
EastFront (639) 1

Biweekly Contest 14:
Zuikaku (96) 1 2 3 4
EastFront (227) 1 2 3 4

Weekly Contest 164:
Zuikaku (166) 1 2 3 4
EastFront (401) 2 3 4
lichunzhu (678) 1 2
AestheticXJTU (822) 3

Weekly Contest 164:
EastFront (401) 1
AestheticXJTU (822) 1

Weekly Contest 163:
Zuikaku (116) 1 2 3 4
EastFront (199) 1 2 3 4
AestheticXJTU (1045) 1 2 3

There should be more; my tool can easily be fooled by renaming variable names and by injecting dead code here and there. Anyone can share a better plagiarism detection logic, one that can at least perform some lexical analysis?

BTW: also, quite often each of those ids used three languages (java c++ python) in a single contest. while it is definitely not impossible, I felt like that there might be two or more people behind those ids. Honestly I can program more than three languages but I only use one during the contests (very rare occasion would I use another one, that is when I really need the treeset).