Of course, if the company is the contractor's only client, the company has to fire them as soon as they know.
That gives the contractor a nice incentive to lie about it.
You might think that the contractor lying would be enough to keep the company safe. In a sane system, that would be true. But labor courts in Brazil are notorious on siding with employees even when it doesn't make sense.
I've even had a friend who lost a case where the judge admitted (off the record) that my friend (the employer) was right, but he was ruling in favor of the employee, since he was so poor.
I also seem to recall that the legislation that made a contractor with a single client legally an employee worked per a yearly period - so if someone worked for you for a few months, and then had no other clients within a fiscal year, he counts as an employee. I can't find a reference right now, though.