Tautology . . . what is it? You could have easily looked this up yourself, by going back through an experienced editorials, or on the internet or in a good dictionary. I am happy to answer your question, but you must get used to doing fast research yourself.
A tautology is needless repetition of an idea or concept. If you wrote He descended down into the mine, you've committed the grammatical crime of a tautology. "Descend down" is silly because "descend" means "down". If you say PIN number, that's a tautology because you're saying "personal identification number number". Today's modern consumer or A summer's day in February in Australia or World champion of this global sport are all tautologies.
- Also a tautology can be a statement that has to be 100% true because it includes all possibilities such as "Well, he's either dead or alive" or "My soccer team will win, lose or draw". This kind of tautology is also called by John Cleese "...a statement of the bleedin' obvious".
Here's more examples of tautologies:
$100 dollars Joint cooperation HIV virus
dot.com In my opinion, I think that... Win a Free Trip!
ATM machine First priority CD-ROM disk
And etc. 2am in the morning Necessary requirement
To reiterate again The reason is because Close proximity
One after the other in succession Female daughters
Adequate enough (it's either "adequate" or "enough")
Please RSVP (that acronym stands for the French "Respondez, s'il vous plait," or, "Respond, if you please.")
My Macquarie Dictionary says:
Tautology: needless repetition of an idea, especially in other words in the immediate context, without imparting additional force or clearness, as in to descend down.