Multiple paternity of offspring can result from active preferences on the part of females or sexual harassment by males. We examined sexual responses of female guppies to a previous mate versus a novel male (experiment 1) or to a male with a colour pattern similar to that of the previous mate versus a novel male (experiment 2). Females showed significantly more sexual responses to courtship by novel males than to previous mates in experiment 1 or to males that resembled previous mates in experiment 2. These results suggest that females discriminate actively against previous mates, and extend this discrimination to males with similar colour patterns to previous mates. This could lead to negative frequency-dependent sexual selection against common colour patterns (a 'redundant male effect'), which could contribute to the maintenance of the extraordinarily high levels of genetic polymorphism in guppy colour patterns.