English has many examples of redundancy. This can be defined as a repetition of information. In fact, redundancy occurs in all languages.
In the plural two carrots, the plural marker -s is redundant. The reason is that the number two already indicates that the noun is plural. This is different from the sentence I see the carrots where the plural marker is necessary to indicate plurality.
In the double possessive a friend of my father's possession is indicated by the preposition of. The form father's is unnecessary. It is sufficient to say a friend of my father. However, the double possessive is sometimes necessary. Compare a picture of my mother with a picture of my mother's. In the first instance we have a picture of a person's mother, but in the second we have a picture which belongs to a person's mother.
In the question Do you like fish? the verb do is redundant. In the sentence You like fish it fails to appear, and in the question it adds no meaning. In informal language You like fish? conveys the same information.
Redundancy is sometimes used for emphasis. This can be exemplified in ha,ha, oh, oh and no, no. This type of emphasis is also often used to indicate surprise (oh, oh), laughter (ha,ha) and refusal (no,no).
Redundancy is common in language. It can be both grammatical and lexical. In the phrase three houses the redundancy is grammatical, but in free gift it is lexical. Lexical redundancy can be avoided, but grammatical redundancy such as three houses is an inherent and necessary part of the English language.