Fatal error "Cannot redeclare template_preprocess_select()" on every attempt to install modules or themes
Mac OS X should have any of these possible reasons:


Your site folder name is mixed case
You cd'd into your site folder name with the wrong case i.e. cd ~/sites/myproject instead of cd ~/Sites/myproject
Your groups are incorrect (use staff not wheel)
Drush is using a different version of PHP from MAMP (Check the output of php --version.)


If you get a redeclare error running drush cc all, check one of the above issues. If sudo drush cc all does not produce the error, it could be one of them. In my case, changing the case of my project from ~/Sites/MyProject to ~/Sites/myproject made the issue go away.

For more in-depth discussion, see:


https://www.drupal.org/node/1305254
https://www.drupal.org/node/1296594