We know that the potential developed at a point by a charge is the negative of the work done to bring a unit charge at that point from infinity. But how does a conducting plate develop a potential? what is the work done?
I can smell some misconceptions in your question. It is better to revise the question after revising the topic on Elelctric Potential thoroughly.
When a conducting plate is charged, it is raised to some potential. Only to bring the first point charge to the plate, the work done is zero, but afterwards, repulsion is developed between the charge brought and the charge already present on the plate and therefore work is required to be done to increase the charge on the plate.
(Please revise the portions well and review the question asked)
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