Does anyone have a reference that explains the technique of doubling of variables as introduced by Kruzkov? It seems to be a necessary tool for contraction estimates when we have weak solutions. However all the papers I have come across merely use this method without giving any explanation of it. 



Doubling of variables method for parabolic equations
[This paper](http://www.sciencedirect.com/science/article/pii/S0022039696901552) by Felix Otto applies the method to quasilinear parabolic equations, and explains the steps in detail. 


