Admins can link user twice with the same id and username

[erfan-goodarzi]

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.