http://www.doctorsnotestore.com. are openly selling fit notes and while they claim that they are for novelty use only, the possibilities are clear. There appears to be nothing illegal in offering these fit notes in the same way that there are similar sites offering duplicate P60's.
The problem is worsened presently as there is a clear window of opportunity, while the regime is new, for employees to abuse the system. Employers simply are not familiar with the system.
It would seem to be employers that offer company sick pay schemes that are most at risk to abuse but it could affect every employer. In particular, fit notes claiming an employee is "unfit to work" may be most open to abuse.
If you do catch an employee submitting a fake fit note this can be classed as gross misconduct.
It may be an opportune time to look at your contracts of employment and to make sure that they give you the right to refer the employee for independent review by a doctor or occupational advisor.
You do not have to accept a fit note at face value so stay vigilant.