Can a contractor freelancer still work for another person?

[Btaliat]

In some situations, freelancers may be hired permanently by some clients. Most of the times, it is always agents that always hire them
In this, they need to make sure they may a valid contract. This will let them know whether they can work for someone else or not. In a situation where they don't make it clear, it may be illegal of them to do otherwise. In fact, they can be sued by their clients.