In 1970 under new coach Don McCafferty, Duncan won a starting position during the season and he intercepted two passes in the team's last four games.

His affable disposition had turned sullen and Colts owner Carroll Rosenbloom was concerned enough to hold some private conversations with Duncan about whether he was having personal problems.

[2] Duncan was traded along with a 1972 fifth-round selection (126th overall–South Carolina defensive back Bo Davies) and a 1973 sixth-round pick (139th overall–Doug Kingsriter) from the Colts to the New Orleans Saints for John Shinners on January 29, 1972.

He joined the Miami Dolphins, coached by Don Shula, who drafted Duncan in Baltimore, later that season, but he was cut from the team.

His marriage ended, he lost between $22,000 and $58,000 in a wig business, and he received treatment for mental health problems and a bleeding ulcer.

[6] Duncan's career yards per kickoff return average was 32.6, which ranked as the highest in NFL history.