Start by naming the Top 10 (incl ties) HR leaders for 1990. For 1991, name the Top 10 assuming the 1990 Top 10 had Retired. For 1992, assume those in the 1990 & 1991 lists had retired, and so on.
Basically - for a given year, for a player to be in that list they must be among the top 10 HR hitters for that year NOT INCLUDING those who would've been listed for a previous year in the quiz. So for 1992, you are looking for players who weren't in the 1990 Top 10, nor those who were in the 1991 Top 10 (a Top 10 which excluded the 1990 Top 10)...and so on. So no player will be listed more than once.