In theory, this is true but in reality it would be very difficult to eat enough bananas to have that effect.
Bananas are pretty high in potassium and so if you ate enough of them you risk ovderdosing (a condition called hyperkalemia). However, you would probably have to eat somewhere in the region of 50-200 bananas in a pretty short time. Its very unlikely you could do that without being sick and thus throwing up all the potassium!
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