Aptana question - Update conflict alert box

Hi all,

Iv just started using Aptana and have a question that really bugging me,

Everytime I save my file after an edit and preview in my browser then go back into aptana I get an Update Conflict alert box saying:

The file ‘youtube/index.hml’ has been changed on the file system. Do you want to overwrite the changes made on the file system?

Can anyone tell me why this alert box might be displaying and how I can fix this?