Serious Eats
How Hazardous is Double Dipping?
Just how much bacteria is transferred when you "double dip," that is, dip a cracker twice in the same condiment? Maybe more than you think—a study at Clemson University found that three to six double dips transferred about 10,000 bacteria from the eater’s mouth to the remaining dip. Although double dipping is "unlikely to be a major public-health threat," you might want to be careful of who you double dip with.