If the meaning of insanity is, as some have suggested, repeating the same action expecting different results, Atlanta forward Ivan Johnson was by definition out of his gourd. Twice in the span of 18 seconds Saturday, Johnson, a one-time Fox Tech standout, challenged Tim Duncan, following an ill-fated turnaround jumper with an ill-fated attempt at a layup, both producing an all-too-familiar outcome. Chalk up two more blocked shots in a remarkable season full of them for the Spurs’ All-Star power forward. At age 36, and in the midst of arguably the greatest run of shot-swatting in his Hall of Fame-bound career, Duncan says the secret behind his longevity as a defensive menace is twofold. “I’m old,” Duncan deadpanned, “but I’m slow.” Blessed with an All-NBA sense of self-deprecation, Duncan swears his shot-blocking prowess to be mostly the product of good fortune. The rest of the league would love to be so lucky. In his 16th season, Duncan has been offering up more rejections than a homecoming queen, totaling 173 in 65 games — all, as he likes to joke, without leaving the floor. Duncan’s average of 2.7 blocks per game ranks third in the league and puts him on pace for the best shot-blocking season by a 36-year-old in league history. Since the NBA began tracking blocks in 1973-74, no player Duncan’s age has come close to that number over a full season. New York’s Patrick Ewing averaged 2.6 blocks in 38 games of a lockout-shortened 1998-99 campaign. Duncan’s average of 3.2 blocks per 36 minutes, meanwhile, marks by far the highest rate of his career. Are we amazed yet? “I guess to some degree, I should say yes,” coach Gregg Popovich said, “just because people in that stage of their career don’t do the things he’s doing.”