In the 1984 Los Angeles Olympics, Puerto Rico’s Madeline de Jesus sustained an injury competing in the long jump and couldn't run in the 4×400-meter relay event soon to follow. In a stroke of genius, she got her identical twin sister, Margaret, who was competing in the long jump, to run in her place, to help the team qualify for the final, which they did.
The head of the Puerto Rican team heard what happened and withdrew the team from the final. Would they have got a medal had they used someone else in Madeline's place in the qualifying heat? Who knows?