Though he said he didn't know anything about the footprints in the new sidewalk, his duplicity, or deceitfulness, was obvious from the cement caking his shoes. His mouth said one thing, his feet said another.
Many words with "du" have meanings with "two" or "duo." Duplicity is from a Latin word meaning "twofold, having two parts." Someone who shows duplicity is two-faced — maybe showing one side in public and another in private — or is just a liar, saying something known to be untrue or misleading. A fraud uses duplicity to gain something with false promises, and someone described as "fake" might use duplicity just to fit in or be accepted.