The criteria of double dissimilarity is one of the tools that critical scholars use to sort through texts to determine which have a higher degree of certainty of going back to the historical Jesus. The idea is that if a saying in the Gospels does not look too Jewish or too Christian, it more likely goes back to the historical Jesus. That is because it is less likely to have been adopted from an existing Jewish text or invented by the church.
The problem with double dissimilarity is that Jesus was Jewish and the Christian church was based on his teachings so we would expect Jewish and Christian sounding statements going back to the historical Jesus.