The issue isn't just the lack of diagnostic - what kind of erroneous
code is being generated in this case?
Let's see what the final C11 draft (N1570) says about this issue:
Section 6.8.4.2 paragraph 3:
The expression of each case label shall be an integer constant
expression and no two of the case constant expressions in the same
switch statement shall have the same value after conversion.
Section 4 paragraph 2:
If a "shall" or "shall not" requirement that appears outside of a
constraint or runtime-constraint is violated, the behavior is undefined.
There you read it, C11 allows us to generate whatever we want.