1) Carbon copy 2) Certain look-alike 3) Double 4) Monozygotic sibling 5) More than a look-alike 
(from the article `behaviour genetics`) ...quasi-experimental methods are used to screen for genetic influence on individual differences in complex traits such as behaviour. The twin method ... Some of the most powerful experiments to dissect the `nature versus nurture` aspects of human intelligence and behaviour have involved...
Twins produced by the division of a single zygote
[n] - either of two twins developed from the same fertilized ovum (having the same genetic material)
one of a pair of twins who develop from a single fertilized ovum and therefore have the same genotype, are of the same sex, and usually resemble each other closely. Cf. fraternal twin.