REDUNDANCY.
1. In general usage, more of anything than is (strictly) needed, usually resulting from REPETITION or duplication; PLEONASM or TAUTOLOGY. In the sentence They also visited us last week too, either also or too is redundant, because both words express the same idea
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2. Technically, both the repetition of information (or the inclusion of extra information so as to reduce errors in understanding messages) and part of a message which can be eliminated without loss of essential information. Languages differ in the degree and kinds of redundancy they make use of: LATIN syntax has a much higher level of redundancy than English syntax. In the sentence Milites novi hodie venerunt, as compared with its translation The new soldiers came today, plurality is marked three times in Latin (-es in milites, -i in novi, -erunt in venerunt) but only once in English (-s in soldiers). FRENCH often has greater redundancy in writing than in speech: in Les nouveaux soldats sont venus aujourd'hui, the plural is carried in speech by les, sont and in writing by les, -x, -s, sont.

Redundancy and information

Redundancy can be described as the difference between the possible and actual information in a message. This difference may be repetition or other encodings beyond the minimal possible length. A message entirely without redundancy may contain the maximum amount of information, but cannot be corrected if it is corrupted in some way, because there is no ‘spare’ material to check with. In the Latin sentence above, three items would have to be lost before the plural message is lost. In the English equivalent, the loss of one -s would change the message drastically. To avoid misunderstandings, people generally repeat themselves more when speaking than writing. This corresponds to the greater possibility of error in listening than reading.