Apparently double dipping does transfer bacteria from the mouth of the initial double dipper to all subsequent dippers.
On average, the students found that three to six double dips transferred about 10,000 bacteria from the eater’s mouth to the remaining dip.
Each cracker picked up between one and two grams of dip. That means that sporadic double dipping in a cup of dip would transfer at least 50 to 100 bacteria from one mouth to another with every bite.
But whether this is a health risk is still debatable.