A next friend is not a privy of the person under disability for res judicata purposes, because the next friend does not 'claim under or through' the person under disability. However, where a counterclaim is in substance a claim by the person under disability (even if brought in the next friend's own name), the court will look to the substance rather than the form and treat the proceedings as between the same parties. Successive professional negligence actions based on different particulars of negligence arising from the same retainer will be barred by res judicata or Anshun estoppel where the later particulars could and should have been raised in the earlier proceedings.
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