I think that you're wanting players to have one account rather than one username (just trying to establish that we're both on the same page there)
One of the most difficult downsides for any game that enforces one account / person, is how you actually define one person from a software perspective.
limit of 1 person per device
This negatively impacts families (e.g two brothers may have separate steam accounts and separate purchases, but only have the one PC)
limit of 1 person per purchase
This can easily negatively impact our younger players especially (quite often if parents play RISK, they may have used their own payment account to set up their child's account) - resulting in separate users with a shared purchase.
limit of 1 person per IP address
Virtually impossible to do. You could technically enforce in use restrictions (only 1 account can be connected from one IP address at a time) but there's a lot of risks involved there too.
Public WiFi networks (e.g Starbucks)
Shared networks (businesses, schools, government as examples)
Mobile IP addresses will change every day or two constantly (any fixed limitations will heavily break for mobile use, many people can share the same IP address at once as the ISP often groups users to one)
Dynamic IP addresses (much more common now than static, these are often recycled too)
Static IP addresses are the only ones that could really be enforced decently (and would result in the results you'd hope for, a small proportion of players though)
VPN use (any restrictions per IP will result in lots of things messing up here)
Additionally it'd just make it harder to track cheating because you'd be removing one of the methods of potential association.
phone verification
This is a little harder to get around, but pretty much anyone can get a SIM card for $1 (so it does have potential forced limits, but not exactly guaranteed) - phone verification is also quite an old form of 2FA that isn't the most secure (it often uses randomly assigned phone numbers to send from, so your message could easily be sent before or after a scam message, eroding trust in using them in the first place) - wouldn't be too bad to add vs limit of 1 account per person
A lot of the potential routes for trying to restrict players to 1 account have little impact on malicious actors (there are ways around anything) - and it tends to just overcomplicate & confuse the process for genuine players.
Wouldn't it be better to just have requirements to access different ranked modes, e.g:
Play 100 games - access to ranked FFA, ranked 1v1, on classic
Play 250 games - access to modifiers in ranked games
Play 500 games - access to any map in ranked games
So these would be different queues, and getting 500 games on each account would be too much trouble to bother with in the first place?