Recently we had noticed that several of our success messages, such as “Product Name was successfully added to your cart”, where actually being added twice. Several other messages were also duplicated we had noticed.
This was working fine on our development server and another upgrade instance. The only difference between our production and development servers is that we run Varnish on the webserver.
Thankfully, one quick Google search, revealed that the plugin we use to coordinate between Magento and Varnish (Available Here), actually had a bug that would duplicate these types of messages.
To fix the issue, you just need to download the latest version of the Messages.php file from the Turpentine github of the plugin site (See Here)
One quick check on the code (to see what changed) and an FTP later, the messages were all fixed.
