Yes and no. What Martin Luther King Jr. did would be considered plagiarism today, no it didn't effect his own writings. Essentially the story is that nearly twenty years after Dr. King's death, his wife Coretta, donated his papers to Stanford University. As they were annotating and examining his dissertation, they discovered portions that had been plagiarized from other materials in the sense that King had failed to use quotation marks and give proper credit to the original authors.