[appveyor] warning on existing whls

appveyor.yml

@@ -71,10 +71,10 @@ deploy_script:
 - echo password:%pip_access% >> %USERPROFILE%\\.pypirc
 - ps: >-
     if ($env:APPVEYOR_REPO_BRANCH -eq 'devel') { 
-        twine upload -r test dist/*
+        twine upload --exists warning -r test dist/*
     }
     elseif ($env:APPVEYOR_REPO_BRANCH -eq 'deploy') {  
-        twine upload dist/*
+        twine upload --exists warning dist/*
     }
     else {
         echo not deploying on other branches