What potential issues can arise when including external files multiple times in PHP scripts?

Including external files multiple times in PHP scripts can lead to problems such as redeclaring functions or classes, causing errors or conflicts. To prevent this, you can use the `require_once` or `include_once` functions instead of `require` or `include` to ensure that the file is only included once.

// Use require_once or include_once to prevent redeclaring issues
require_once 'external_file.php';