Describe the bug
If a user visits /locallogin.php with an active local session and alternateloginurl is set in such a way, that /login/index.php makes a redirect to an identity provider, an active session actually prevents the user from continuing to the moodle system.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Site administration' -> 'Plugins' -> 'Authentication' -> 'Manage authentication'
2. Verify that authentication method 'Shibboleth' is active.
3. Verify that 'alternateloginurl' is set to '/auth/shibboleth/index.php'.
4. Go to 'Site administration' -> 'Plugins' -> 'Authentication' -> 'Shibboleth'
5. Verify that Shibboleth is configured properly and especially 'alt_login' (Moodle WAYF service) is set to 'No'.
6. Go to 'Site administration' -> 'Appearance' -> 'Boost Union' -> 'Look' -> 'Login Page'
7. Verify that 'loginlocalloginenable' is set to 'No'.
8. In a new browser window (no session), go to '/theme/boost_union/locallogin.php'
9. Login with a manual account.
10. Verify that you have a valid session (i.e. are logged into Moodle).
11. Go to '/theme/boost_union/locallogin.php'
12. You should be redirected to the IdP of your Shibboleth configuration.

Expected behavior
The expected behaviour is for the user to be redirected to the appropriate starting page for a user with a valid session. This is done via /index.php, not /login/index.php.

Screenshots
n/a

Desktop (please complete the following information):

• OS: irrelevant
• Browser: irrelevant
• Version: irrelevant

Smartphone (please complete the following information):

• Device: irrelevant
• OS: irrelevant
• Browser irrelevant
• Version irrelevant

Additional context
This feature is not a core feature of Boost Union, yet very useful for sites where a single IdP is used via the auth plugins login page in alternateloginurl. Still, this combination of circumstances creates unexpected behaviour and thus qualifies as a bug. (imho)