Students at Clemson University decided to test the issue brought up in an old episode of Seinfeld: Is double-dipping unsanitary?

In short: Yes.

One exerpt:

"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. "

Gross. No double-dipping, people!!

link: read the whole story here