What is tautology and redundancy in English grammar?
Tautology and redundancy are essentially the same thing: saying the same thing twice in succession.
It is important to distinguish between tautology which is intended to improve the style of a piece of writing and tautology that is unintentional.
Most tautology comes under the second category, but there are times where tautology can be used intentionally. One example is a free gift. A gift, by definition, is free, but the repetition is used to indicate the lack of a fine print or any hidden conditions.
Some examples of unintentional tautology include HIV virus and ATM machine which are written out as human immunodeficiency virus virus and automated teller machine machine.