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That's OK.  The language reference manual has always been 
careful to say that an expression returning an immutable 
object may or may not return a pre-existing object.

"for immutable types, operations that compute new values 
may actually return a reference to any existing object with 
the same type and value, while for mutable objects this is not 
allowed"

So, e.g., had your function been

def f():  return ()

it was already true, under CPython, that f() is f().  No 
correct Python program could rely on that, though; neither 
on that only the empty tuple has been shared until now.