To reproduce:
1) Create a local branch named master
2) Create a change and upload it
3) Checkout a new branch with the local branch named master as the upstream branch.
4) Create a change and upload it

What happens:
A duplicate of the parent CL is created.

What should happen:
The parent CL should become the parent of the second change---a local branch named master should be treated like a local branch not as the remote master branch.