Why is this an issue?

Having all branches of an if chain with the same implementation indicates a problem.

In the following code:

if b == 0:  # Noncompliant
    doOneMoreThing()
elif b == 1:
    doOneMoreThing()
else:
    doOneMoreThing()

b = 4 if a > 12 else 4  # Noncompliant

Either there is a copy-paste error that needs fixing or the unnecessary if chain needs removing.

Exceptions

This rule does not apply to if chains without else.

if b == 0:  # no issue, this could have been done on purpose to make the code more readable
    doOneMoreThing()
elif b == 1:
    doOneMoreThing()