Spoiler Alert:
The idea of the 7 Harry Potters was planted into Mundungus' head by Snape (on Dumbledore's orders). He had to give the real date of Harry's relocation to Voldemort in order go gain his trust (even more), but he confunded Mundungus so they had a good chance of getting Harry out succesfully. (Voldemort wasn't expecting 7 Harry's).
Följande är det relevanta utdraget från Harry Potter och The Deathly Hallows
“You will have to give Voldemort the correct date of Harry’s departure from his aunt and uncle’s,” said Dumbledore.
“Not to do so will raise suspicion, when Voldemort believes you so well informed. However, you must plant the idea of decoys; that, I think, ought to ensure Harry’s safety.
Try Confunding Mundungus Fletcher. And Severus, if you are forced to take part in the chase, be sure to act your part convincingly. . . . I am counting upon you to remain in Lord Voldemort’s good books as long as possible, or Hogwarts will be left to the mercy of the Carrows. . . .”
Now Snape was head to head with Mundungus in an unfamiliar tavern, Mundungus’s face looking curiously blank, Snape frowning in concentration.
“You will suggest to the Order of the Phoenix,” Snape murmured, “that they use decoys. Polyjuice Potion. Identical Potters. It is the only thing that might work. You will forget that I have suggested this. You will present it as your own idea. You understand?”
“I understand,” murmured Mundungus, his eyes unfocused. . . .