In fact, the real Thomas "Dipper" Pines died May 23, 2012 and nothing could "cancel" his death. Also, someone tried to kill Mabel too, but Mabel survived and eventually changed her surname to "Desman". Mabel's son Thomas Desman was 42 years old when Bill Cipher came to our world in this timeline, and Thomas successfully defeated Cipher and his minionesm because of Desman's genius mind, but process of destruction of the reality turned out to be irreversible, and then Thomas decided to fix it in the past. He arrived to day of his uncle's death, took his appearance and advised his "parrents" to send him and Mabel to gruncle Stan. For more safety, Tom's memory was erased and replaced by Mabel's brother memory.
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