The phrase proof positive is a redundancy. Proof means evidence or an argument that shows a statement to be true. Because proof confirms the statement in question, positivity is already implied, and the word positive is not needed.
Examples
In these sentences, the word positive serves no purpose and could be dropped:
So, is the massive dumping of snow from the Mid-Atlantic to New England proof positive that climate change is untrue . . . ? [LA Times]
It’s proof positive that if the pond you pick is small enough, a fish of any size looks really big. [NY Magazine]
The signing of Robbie Keane . . . was proof positive that Los Angeles are going all out for the MLS Cup this season. [Vancouver Sun]
