Is this a real thing? It would be essentially identity fraud and having more than a couple of jobs linked on PAYE at the same time would likely raise a flag somewhere in the government
Yes - people are getting tax bills from HMRC after selling their national insurance numbers to dozens of people for them to work illegally, not realising they will be on the hook for the tax which will the affect their benefit claims (child tax credits, etc.)
But then the government should also know where to find all these unauthorised workers, I can't imagine how could anyone pull this off for more than a couple of months without getting arrested (or at the very least losing their job)