Override pluggable functions in a plugin?

Wrap your function in if( ! function_exists( 'wp_authenticate' ) ) to get rid of the error and successfully activate your plugin:

if( ! function_exists( 'wp_authenticate' ) ){
    function wp_authenticate(){}
}

This is necessary because in the context of activating a plugin, the function does already exist, only after it is activated will your plugin load first and override the original in pluggable.php. Plugins are activated in a sandbox to capture any fatal errors which may prevent activation and successful recovery to a working state.

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