Originally Posted by
rankourabu
dont know if this is the case, but you also cannot have CPH-AAA-MLA-AAA-XXX, you cant transfer AAA twice
The rules is much more complicated than that.
xxx-YGK-xxx is not possible without transiting YYZ twice, for example, and SFO-YYZ-YGK-YYZ-SFO is certainly valid.
I don't think you can transit through a stopover point, and there may be an issue transiting the same city twice on one side of the destination.