Not really. If you say "He is easily duped." then you have passive voice. However, if you really want to use it as an adjective, add a hyphen. "The easily-duped guy handed me his wallet." I'll add that this is a really unusual use of the expression.

I guess technically the two words "easily duped" could be, collectively, considered as a single qualifier/adjective, as in "She is easily duped". But strict grammar would dictate that "easily" is an adverb, and "duped" is a past participle.