I have set user registration to 'Admin' on one of my client websites. There are a number of people responsible for verifying users and clicking the activation approval link.

They have mentioned that if they try to click the link after somebody else has already verified the user they receive:

Error 403 - verification code not found
This means that the user has already been verified by another employee.

This is totally normal and what's supposed to happen I understand.

My question - is there a way of changing this error and displaying something like 'This user has already been verified on xx-xx-xxxx' Or something similar.

Joomla 3.3

Using RSForms Pro extension for user registration form.

The scenario you're trying to achieve (add the date when the user was activated) would require solid scripting knowledge to modify the component's source code. An easier approach would be to customize this message to be more user friendly by creating a Language override for the following constant : COM_USERS_ACTIVATION_TOKEN_NOT_FOUND.

Information on how to create a Language override can be found here:

http://docs.joomla.org/J3.x:Language_Overrides_in_Joomla