How can we avoid staff users overwriting emails of utility WordPress accounts when testing CiviContribute pages?
You should also consider giving your staff their own personal user accounts for a number of reasons.


It will be clear that they're acting as themselves rather than as a generic staff person
You can isolate the culprit(s) who are recording memberships the wrong way (see Jon's answer), meaning they don't cause problems for others and you can talk with them specifically
When people leave the organization, change roles, etc. you don't need to change the password for everyone