Bug? Cannot redeclare a pluggable function

I just upgraded to 2.8.2 and now a plugin I wrote has broken:

Plugin could not be activated because it triggered a fatal error.

Fatal error: Cannot redeclare wp_authenticate() (previously declared in […]/wp-includes/pluggable.php:456) in […]/plugins/myplugin/login.php on line 64

Is this a bug? Or am I doing something wrong?

For now I’ve just commented out wp_authenticate in pluggables.php, but that’s not a very good solution!