The new launcher creates a random UUID which is stored as the clientToken in the launcher_profiles.json file. As far as I can tell there is no system in place to prevent two clients from generating the same UUID.

When two clients log in with the same UUID, the first client's session gets invalidated, preventing him from joining any servers. The error message displayed is:

Failed to login: Bad login

It should be possible to log in to multiple accounts with the same UUID or there needs to be a system to prevent collision. (There are some pieces of code in the launcher suggesting that such a system is in the works.)

Steps to reproduce:

• Start client 1 on computer 1
• Log in on client 1 using account 1 (don't use the auto-login feature)
• Copy the generated launcher_profiles.json from computer 1 to computer 2
• Start client 2 on computer 2
• Log in on client 2 using account 2
• Join a server with client 1

(This also works by just starting two client instances on the same computer, they will inevitably share a launcher_profiles.json file and therefore the UUID.)