how to check if custom post type column already exists?

Question

You should first check, and then assign your custom column.
$column should really be $columns to convey the real meaning, as it’s an array.

So, you need to check if the array key exists before the assignment with something like isset() or array_key_exists().

Finally, since your site_title_column() function modifies the value of a filter, you should always return a value (usually the original, perhaps modified), as simply calling return; returns NULL (which is rarely what’s needed).

I’d rewrite your function as such:

function site_title_column( $columns ) {
    if ( ! isset( $columns['views'] ) ) {
        $columns['views'] = 'Title';
    }
    return $columns;
}