Fatal error: Cannot redeclare blah_function() in ../modules/blah.module

When using Drupal your browser returns the following error

Fatal error: Cannot redeclare blah_function() (previously declared in /modules/blah.module:blah) in ...blah... on line blah

Often occurs on:

• admin/modules
• admin/themes
• after modifying module or theme files
• after duplicating module folder
• after upgrading the module and forgot to remove old files

To fix this, select one of the possible causes listed below.