Noun

1. A base hit on which the batter stops safely at second base
2. A stand-in for movie stars to perform dangerous stunts
3. Someone who closely resembles a famous person (especially an actor)
4. A quantity that is twice as great as another
5. Raising the stakes in a card game by a factor of 2

Adjective

1. Used of flowers having more than the usual number of petals in crowded or overlapping arrangements
2. Used of homologous chromosomes associated in pairs in synapsis

Verb

1. Increase twofold
2. Hit a two-base hit
3. Bend over or curl up, usually with laughter or pain
4. Do double duty; serve two purposes or have two functions
5. Bridge: make a demand for (a card or suit)
6. Make or do or perform again

Adverb

1. Downward and forward
2. Two together
3. To double the degree