longman:

twice: adverb, predeterminer  
1. two times
He was questioned by police twice yesterday.
twice a day/week/year etc (=two times in the same day, week etc)
Letters were delivered twice a week only.
None of our dinner menus are exactly the same twice over .
2. two times more, bigger, better etc than something else
twice as many/much (as something)
They employ 90 people, twice as many as last year.
twice as high/big/large etc (as something)
Interest rates are twice as high as those of our competitors.
twice the size/number/rate/amount etc
an area twice the size of Britain
double adj. v. n. adv. predeterminer
adj
twice as big, twice as much, or twice as many as usual
a double whisky The city was enclosed by walls of double thickness.
predeterminer
twice as big, twice as much, or twice as many
double the amount/number/size etc
We'll need double this amount for eight people.The value of the house is double what it was.