Drupal's locale module includes a lot of great features for supporting multilingual sites. One such feature is the ability to associate a language with a path alias. This allows you to have one node with two versions (let's say an English version and a Spanish version)--each with its own alias.
But your use case may not require language-specific paths per node. Maybe you want to call a spade a spade-- you've got a Spanish node or and English node and that's it. No fancy multiple versions.
Well then you've got a bit of a problem--a few actually. This can wreak havoc with aliases, and pathauto in particular. The solution is the Local Path Ignore module. It ignores the language of a given path, and instead forces all paths to have a language of "undefined," effectively allowing path aliases to behave the way you'd expect--one path per node.