Employees should be empowered to the best job ever. Thinking you can do the absolute best at everything, is something we often long to see in employees who lack confidence. If the employee is not really doing a better job, don't discourage them from trying to do a better job, or from thinking that they can over come each challenge.
Your question, doesn't say that the employee thinks that they can do -your- job better, but "a job". This may simply be a task that you both do or a task that you may be able to delegate.
My advice is that you shouldn't be concerned with employees who think they can do a job better than you. Just do the best job you can without being afraid of passing on your experience to someone who can use it to their advantage, so that they can do a better job. You never know what you could learn from your subordinates. Ask for their feedback, they may have valuable insight you may not have considered. As a manager, you need to be able to recognise the people in your team who can succeed you. Give them a succession plan, motivate and empower otherwise you could risk loosing a valuable employee.