validate

This module performs additional validation when a node is created or updated by a user to require that a node's title, author, language, and CCK fields are unique within a given context.

All those are configurable options.

Search words:
drupal prevent duplicate submissions
drupal prevent duplicate node titles
drupal deny duplicate content

I was getting all excited about writing this module when I finally found it, at the last second, while brainstorming titles for it. (No, unique field

Provides a simple FAPI item that can be used to skip validation on required fields when a pre-specified form button is clicked.

This also provides an example module which disables required validation checks on node preview and delete.

This could just maybe be something I could steal some ideas from to do my long-dreamed of validation error override (when forms time out or something and tell you to contact the administrator).