Tautology, often regarded as a fault of style, was defined by Fowler as “saying the same thing twice”. In fact, it is not necessary for the entire meaning of a phrase to be repeated; if a part of the meaning is repeated in such a way that it appears as unintentional or clumsy, then it may be described as tautology. On the other hand, a repetition of meaning which improves the style of a piece of speech or writing is not usually described as tautology.

Redundancy, in general terms, refers to the quality or state of being redundant, that is: exceeding what is necessary or normal; or duplication. This can have a negative connotation, especially in rhetoric: superfluous or repetitive; or a positive implication, especially in engineering: serving as a duplicate for preventing failure of an entire system.

redundancy means supply of useless information

redundancy — overflow , copiousness , superfluity .

Redundancy: Supplying unnessesary information.