Creating this as a separate issue so as not to delay incorporation of the accepted PEP.

One legitimate criticism of the accepted PEP 435 is that the combination of requiring explicit assignment of values *and* allowing aliasing by default is that aliases may be created inadvertently. I believe we can actually do better than the initial implementation by making the following work:

>>> class Shape(Enum):
...   square = 2
...   diamond = 1
...   circle = 3
...   alias_for_square = square
...
>>> Shape.square
<Shape.square: 2>
>>> Shape.alias_for_square
<Shape.square: 2>
>>> Shape(2)
<Shape.square: 2>

While *disallowing* the following: 

>>> class Shape(Enum):
...   square = 2
...   diamond = 1
...   circle = 3
...   alias_for_square = 2
...

How, do you ask? By wrapping non-descriptors on assignment in a placeholder type, and keeping track of which values we have already seen. If a new attribute is mapped to a placeholder, then that's fine, we accept it as an explicit declaration of an alias. However, if it's mapped directly to a repeat value, then that would be disallowed (as it was in earlier versions of the PEP).