To the faithful in danger of death, who cannot be assisted by a priest to bring them the sacraments and impart the Apostolic Blessing with its plenary indulgence (see Canon 530 ß 3 of Code of Canon Law), Holy Mother Church nevertheless grants a plenary indulgence to be acquired at the point of death, provided they are properly disposed and have been in the habit of reciting some prayers during their lifetime. The use of a crucifix or a cross to gain this indulgence is praiseworthy.
The condition: provided they have been in the habit of reciting some prayers during their lifetime supplies in such cases for the three usual conditions required for the gaining of a plenary indulgence.
The plenary indulgence at the point of death can be acquired by the faithful, even if they have already obtained another plenary indulgence on the same day.
The above grant is taken from the  Apostolic Constitution The Doctrine of Indulgences, Norm 18.