what is tautology ?

The term "tautology" can be very briefly explained as superfluous repetition of the same meaning in different words. For instance ,necessary esentials.

A tautology is an unnecessary or unessential (and sometimes unintentional) repetition of meaning, using different and dissimilar words that effectively say the same thing twice (often originally from different languages). It is often regarded or thought of as a fault of style and was defined by Fowler as "saying the same thing twice."

Eg : free gift

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Saying same things repeatedly in different words is tautology. This may be for emphasizing something by repeating in different words.