We see MJ wearing the broken black dahlia necklace Peter previously gave to her throughout the movie. In the final scene, where Peter goes to see MJ after he's been erased from everyone's memories, including hers, she's still wearing the necklace. It's unclear how she could have the necklace (or how she thinks she got the necklace) if Peter never gave it to her, which opens up another massive plot hole about how Strange's spell actually works.