Fool me once, shame on you; fool me twice, shame on me.
Prov. After being tricked once, one should be wary, so that the person cannot trick you again. Fred: Would you like a can of peanuts? Jane: The last can of peanuts you gave me had a toy snake in it. Fred: This one really is peanuts. Jane: Fool me once, shame on you; fool me twice, shame on me.