Tautology - keep it out of writing.

Something I saw on a writing website I frequent:

Tautology is the use of words that duplicate the meaning of a word or words already used, as in:

basic principles
hollow tube
mutual cooperation
personal opinion
exactly equal
still continues
past memories
various differences
each individual
true facts
important essentials
future plans
terrible tragedy
end result
final outcome
free gift
past history
unexpected surprise
sudden crisis

An expression that's not redundant as much as it is illogical is "very unique." Since unique means "one of a kind," adding any modifier of degree -- very, so, especially, somewhat, extremely, etc. -- is illogical.