Do tópico Perguntas dos desenvolvedores sobre o tópico nos fóruns do FFG (encontrado dentro do Colapso Geral de Combate ):
Rules as written a character would not suffer a second Critical Injury if the character takes more damage and is already incapacitated.
However, since the character is incapacitated and cannot defend himself, the GM would be perfectly within his rights to allow the second hit to inflict an automatic Critical Injury, or any other nasty effects depending on the circumstances.
If the character is being shot at point blank range while lying helpless on the ground, for example, the GM could simply skip the Critical Injuries altogether, and proceed directly to complete, unrecoverable death.