Definition of impolite in OD:
not having or showing good manners; rude.
Definition of unpolite in TFD:
Not polite; impolite; rude.
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The key difference is that unpolite is now archaic/incorrect and so should be avoided, though it was once the more common form (see Google Ngram Viewer). When both were still in use, it appears they were largely synonymous. For example, Webster's 1828 dictionary defines them as follows:
IMPOLI'TE, a. Not of polished manners; unpolite; uncivil; rude in manners.