Cannot redeclare class Drush error
What version of Drush are you using?  Recent versions of Drush will notice that there is a copy of Drush included in your Drupal site, and will redispatch (exec) to use it instead.  If you are using an old, unsupported version of Drush (e.g. Drush 6 or earlier), then it will not do this, and the error you see above would be the likely result.