There is no such word. You probably mean "tautology". A tautology, in formal logic, is a statement which is always true because of its structure, for example: A=A. By extension, "tautology" can be used to describe a phrase that unnecessarily repeats the same idea in different words, such as "free gift", or "true fact", or "2 a.m. in the morning". This is also known as a redundancy.

an identity?

maybe a tautology?

Comment by mgately:

In the field of discrete mathematics (simplified the study of logic) any expression which always evaluates to true is in fact called a tautology. While less cool sounding, an expression which always evaluates to false is just called a contradiction.

Tautology is the useless repetition of words.

I am going to the mall or I am not going to the mall is a tautology.

Tautology is not simply the useless repetition of words. It is more about redundancy. The example above is tautology but it is because the phrase is redundant. "I may go go the mall today." implies that I may not to to the mall today. To include that I may not would be tautology.

Another example of tautology is when you have two words whose meaning is the same used in conjunction. "Free gift" and "unsolved mystery" are tautology. The words are synonymous and therefore they are redundant.

Doors. Although the question probably should be 'What are all chairs?' to make it an exercise in deductive logic rather than tautology :)

The person kept saying the same thing over and over which had no meaning so it was tautology.