For example: tautologic, tautological, and tautologous.
closed as general reference by Matt Эллен, MετάEd, TimLymington, kiamlaluno, tchrist Sep 4 '12 at 1:48
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For -ic vs -ical see a Metrolingua entry that basically states that most uses overlap almost entirely, but in some cases there is a distinction, as in economic vs economical, where economical implies thrifty, and economic really just refers to anything relating to economy. I don't know if this applies to any examples of the -ogous suffix.
The article further describes an adjective politic that meands prudent, and historic, which means a significant event in history, not just any event.
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They're effectively synonyms, but most people use tautological most of the time...