When making a new definition, I tend to hit = before hitting : just to make sure that it does not already exist. It is very easy to overwrite a definition, especially in a big document. Sometimes I need to cut and paste large sections from one document to another. I then have to go through and check that there are no conflicting definitions. An incredibly useful feature would be some sort of macro that would summarise all the definitions in a document and highlight duplicates.
Alternatively a document option where you can choose to disallow redefining a definition.