Tautology

Tautology (Greek, 'saying the same thing') is the repetition, in a single statement, of the same idea in different words. 'A free gift.' 'Advance planning.' 'The reason is because.' 'It is what it is.' In most contexts tautology is a fault of style: the second phrase adds no information the first did not already contain, and the sentence is longer than it needs to be.

There are, however, tautologies that do work. Rhetorical tautology — 'boys will be boys', 'it is what it is', 'que sera, sera' — trades on the fact that the surface redundancy carries an implicit second meaning. 'Boys will be boys' does not mean literally that boys are boys; it means, roughly, that certain behaviour by young males is to be expected and tolerated. The tautology is a signal that the speaker is refusing to argue further.

In logic, a tautology is a statement that is true by its form alone: 'either it will rain tomorrow or it will not'. This use of the word is technical and does not carry the pejorative sense the word has in style guides. In everyday writing, when someone objects to 'a tautology', they almost always mean the redundant kind.

The rule for writers is simple. If the second phrase does not add meaning, cut it. If the second phrase looks redundant but is doing rhetorical work — indicating resignation, refusal, or shared knowledge — leave it, and make sure the surrounding context makes the intention clear.