Based on the one piece manga. Will resolve to yes if it is revealed that one of the previous appereances (before chapter 1125) of Shanks was in reality another character with a similar appearance. An example would be if it wasn't Shanks that showed up in chapter 907 to speak with the gorosei.
At least one of the PREVIOUS (before 1125) appearences of Shansk must be by the doppelganger. So if Shanks has a doppelganger who hasn't been seen before 1125, this would not count. If the doppelganger can only temporarily assume the apperance of Shanks, this would count as long as the reader has been misled by the apperance. Doesn't matter if the doppelganger is a twin, a brother a clone, a devil fruit ability, etc.
Will resolve to No when one piece conludes at the latest but I intend to resolve it earlier if the good reasons for believing this has been debunked and new good reasons have not appeared. For instance, let's say the following things happen
We found out that chapetr 907 featured the real Shanks,
We get an explanation of the Figgarland family tree but without any characters that seem to be the doppelganger.
We find out the identity of the silloute at the end of chapter 1121 which isn't the doppelganger.
If no similiarly strong indications of a doppelganger are presented in the comments section I would resolve the question to No after 1 month.
More specific prediction based on the same mystery: https://manifold.markets/YODAOV/does-shanks-have-a-twin