A person who vapidly, unthinkingly yet wholeheartedly repeats, shares and takes to heart phrases and ideas he/she hears from his perceived leader. Most often (OK, damned near exclusively) applied to followers of right-wing media magnates like Rush Limbaugh, Michael Savage Weiner, Sean Hannity, Bill O'Reilly, et al. A ditto monkey is often a raging dullard who can't normally put two syllables together without a paper clip, yet when the topic of their idol's specialty comes up (usually politics), his eyes glaze over and he eloquently spews words you know he couldn't otherwise possibly understand.
"Oh great... E-mail from Patrick. Hideous spelling, every fourth word in ALL CAPS, punctuation looks like a typewriter threw up, and he's blaming the weather on the Clintons. What a flippin' ditto monkey!"