Error When Redefining Pluggable Function

I am trying to create a plugin to modify the wp_mail function in the pluggable.php file. I understand that to do this I simply need to define it somewhere else and WordPress will handle the rest.

If that’s the case, why do I get this error when I activate my plugin?

Fatal error: Cannot redeclare wp_mail() (previously declared in .../wp-includes/pluggable.php:271)