Pointing my own domain to a shared instance via WebFinger .... it works but

So … I have managed use use my custom domain and make it point to a shared instance, without hacking Mastodon
… and, it kinda works …

I have developed a small python app webfinger-proxy to alias your own domain handles to a shared instance.

I have setup it on my own domain rjo.name.
The handle testrjo@rjo.name points to testrjo@mamot.fr
See the response of the Webfinger https://rjo.name/.well-known/webfinger?resource=testrjo@rjo.name

I have managed to follow this account and send it private messages.
I have another account on the same shared instance, and testrjo@mamot.fr appears as a completely separate remote account though.

I am not sure whether this is only on the UI or if Mastodon now have a duplicate account for this guy …
I would need some help from the developers to figure this out : Is the handle a primary key ? Or is it the URL ?
Can you see any have adverse effects ?
Do you think the physical migration of accounts from one instance to another would still work ?

I will continue my tests with other implementations :
Pleroma, mobile clients, etc …