No, "undue" is not a preposition. It is an adjective used to describe something that is excessive or unwarranted.
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Undue
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Duress and undue influence are pressure put on someone to do
what is being requested of them against their best wishes
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Undue prejudice is when the defendant is not able to get a fair
trial. Undue prejudice can occur when there is a lot of media
coverage, or the jury has been turned against a defendant because
of the nature of the crime he is accused of.