Should avoid start two debug sessions for one debug configuration

[andxu]

We should avoid two start request during startup.

[akaroml]

This is the correct behavior. We don't limit a user to only work with one debug session.
Problem is how you started two sessions.

[andxu]

During language start up, press F5 two times.

[yaohaizh]

If seems when the language server is still initializing, when press F5, our code is still not calling yet. Maybe a n issue of VSCode.

[yaohaizh]

Fixed.