Do we have to redo trials if they are outliers?

Say I'm doing 5 increments of whatever independent variable, and 5 trials for each increment. I get an outlier on trial 3 for an increment. do I do a 6th trial for that increment or do I just ignore it when it comes to averaging out my data. And if i do decide to redo the trial, do i keep a record of the outlier or just act like it never happened in my IA?