REALITY Who Do You Think You Are? CBC, 8:30 p.m. Some TV ideas just keep coming back. Launched with great fanfare by CBC for the 2007-2009 broadcast season, this homegrown take on a popularBritish series ran one short season before it was pulled, presumably due to low ratings, and then shelved until tonight. As with the recent successful NBC spinoff, the format follows famous people as they track their the roots, which allowed Canadian viewers unprecedented insight into the family trees of Don Cherry, Randy Bachman, Margaret Trudeau and Mary Walsh, among others. The second season opens with a profile of Don Cherry, whose genealogy has presumably evolved since the first series.