I have also tried this in survival but the same outcome. I put a hopper on top of another hopper and the top hopper doubles in speed.
Linked issues
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This seems to be the same issue/bug as MC-6462
MC-7556
Hopper speed increasing
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Theory #1:You have both of these hoppers going to chests.
If that is the case then you don't have a bug. When you place a stack of items in the hopper your top hopper is going to be sending items to it's assigned chest. while the lower hopper is going to be pulling items from the top hopper to put into the lower chest.
Theory #2: your top hopper is pointing into lower hopper.
What would be happening here is that your top hopper is sending items to lower hopper while lower hopper is pulling from top hopper. If that is the case then you still don't have a bug, but rather a potentially unexpected behavior of hoppers.