Admins can link user twice with the same id and username

## Issue

In the current implementation of `link` command we can link a github account to the same telegram username or telegram id twice. It's better to show hint and avoid linking twice.

## Scenarios
- When admin use `link` command directly and then use it twice.
- When admin use `link` command by replaying to a user then use it twice.
- When admin use `link` command directly and then replaying it to the same user twice and the opposite.