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I'm not sure anyone advises Hamlet to repent. Claudius and Gertrude want Hamlet to quit looking so gloomy and "look like a friend on Denmark." Hamlet asks his mother to repent her sins much later in the play. Perhaps if you are more specific, I can help further.
Queen Gertrude is advised by Hamlet to repent. He wishes her to repent, and afterwards he plans to seek her forgiveness.
I can't think of a line where Gertrude asks Hamlet to actually repent. Do you know where the line is?