Pujols tied a season high with five R-B-Is after entering the game in an 0-for-11 skid. He was hit by a pitch with the bases loaded in the fifth, one batter before Duncan smacked his second grand slam of the year.
Scott Rolen also homered for the Cards, who kept the Cubbies two games behind the N-L Central-leading Milwaukee Brewers.
Braden Looper evened his record at 8-and-8, holding the Cubs to a run and five hits in seven innings. He blanked Chicago until Derrek Lee homered in the sixth.
Former Cardinal Jason Marquis was scorched for six runs and five hits in five frames. He's 7-and-6.