Where, of three or more concurrent wrongdoers, one is omitted from the claim for contribution, contribution shall be awarded to the claimant on the basis that responsibility for the damage is to be borne by the claimant and the contributor or contributors without regard to the responsibility of the omitted wrongdoer, and, in such a case, a claimant whose net remaining liability is increased or a contributor whose contribution is increased by reason of the fact that judgment has not been given against the omitted wrongdoer may claim contribution from such omitted wrongdoer in accordance with the provisions of this Part: but, where such last-mentioned claim for contribution is made by a contributor in respect of his own liability to make contribution and judgment is given in such contributor's favour, execution shall not be issued on that judgment -
(a) except in accordance with the provisions of section 23, with the substitution of "contribution" for "damages" and of "original claimant
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