add skip-existing

CONTRIBUTING.md

@@ -159,7 +159,6 @@ pytest
 ```bash
 pip install --upgrade pip
 pip install -r build_requirements.txt
-rm -r dist || true
 python -m build
 ```
 
@@ -167,7 +166,7 @@ python -m build
 
 ```bash
 pip install --upgrade twine
-python -m twine upload --repository pypi dist/*
+python -m twine upload --repository pypi dist/* --skip-existing
 ```
 
 You will be prompted for a username and password. For the username, use __token__. For the password, use the token value, including the pypi- prefix.