dir() gives duplicate results for methods

Reported by @tmcdan on Slack, and independently confirmed by @k2bd.

```
import traits
from traits.api import HasTraits

class A(HasTraits):
def foo(self):
print(a)

print(traits.__version__) # 5.0.0
print(len([d for d in dir(A()) if d == "foo"])) # 2
```