due vs undue

due is a noun but undue is not a noun.

Word	Noun	Adjective	Verb	Adverb
due	Yes	Yes	No	Yes
undue	No	Yes	No	No

due (noun)	undue (noun)
that which is deserved or owed
a payment that is due (e.g., as the price of membership)

due (adjective)	undue (adjective)
owed and payable immediately or on demand	not yet payable
suitable to or expected in the circumstances	not appropriate or proper (or even legal) in the circumstances
capable of being assigned or credited to	beyond normal limits
scheduled to arrive	lacking justification or authorization

due (adverb)	undue (adverb)
directly or exactly; straight