I don't know of the 'Duplicate' tool in the Vanishing Point dialog. Do you mean the "Stamp Tool"? If you mean the stamp tool, have you alt/opt click to set the source point? Is the Opacity of the tool set to 100%?
Thanks for your answer, yes it is the "Stamp Tool" :) I would try to send you a small video because Alt/opt does not work !! When I click the Alt/opt key to set the source, I just have a big black arrow !! Otherwise the opacity is 100%