Tautology means to repeat your meaning superfluously with unnecessary words, as in the following sentence:

In actual fact, there is a consensus of opinion, backed up by past history, that it is absolutely essential that we do not overexaggerate the positive benefits at this moment in time.

For instance, it is tautologous to say positive benefit as a benefit is always positive. Similarly, history already includes the meaning of past. Some examples of tautology are given below:

Tautology Better
absolutely essential essential
advance warning warning
at this moment in time at present
blue in colour GB or color US blue
a consensus of opinion a consensus
cool down cool
divide up divide
few in number few
future prospects prospects
heat up heat
in actual fact in fact
join together join
overexaggerate exaggerate
past history history
positive benefit benefit
previous or past experience experience
prewarn warn
revert back revert