I never fear being fooled.
It happens occasionally, a side effect of believing in each person's potential. There are people who see kindness as weakness, and while it's better to steer clear of such people, some hide it well.
But each time, I learn from my mistakes and move on.
And if the person comes back, asking for a second chance, I don't feel guilty when I say, "no."
Second chances are meant for mistakes, not duplicity.