When one asks for explanation of something, the other give an equivalent explanation. Examples: "the weather is hot because it is not cold", "I am smart because I am not stupid". So what is the common idiom to describe this case?
I can think of no idiom that directly captures a tautological utterance itself. Not unless you want to imbue the tautology with a nonsensical meaning. In which case, there's A nod is as good as a wink (to a blind horse / to a blind bat).
But there is an expression that captures a response to a tautology.
"It's hot because it's not cold."
"I don't care what you call it—I still have to wear a T-shirt."
The response can be captured with: A rose by any other name would smell as sweet.
This is the common quotation from William Shakespeare's Romeo and Juliet. The actual two lines, as quoted from BookBrowse, are:
What's in a name? that which we call a rose
By any other name would smell as sweet
In comments below your question, you said you weren't interested in anything related to reasoning or fallacy, just an idiom that expressed the idea of "the saying of the same thing twice in different words."
This expression can have a broader interpretation than just a tautology, but it can be used in the case of a tautology.