Corroboration

n. reinforcement; confirmation; support

Corroborating evidence is evidence that tends to support a proposition that is already supported by some evidence. For example, W, a witness, testifies that she saw X drive his automobile into a green car. Y, another witness, testifies that when he examined X's car later that day he noticed green paint on its fender.
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Noun
1. confirmation that some fact or statement is true
(synonym) documentation, certification
(hypernym) confirmation
(derivation) corroborate, underpin, bear out, support

n. corroboration

(n.)
The act of corroborating, strengthening, or confirming; addition of strength; confirmation; as, the corroboration of an argument, or of information.
  

(n.)
That which corroborates.