Atlassian products will no longer allow installation of a local app in development mode at "Manage apps" page, which uses the same key as an installed Atlassian Marketplace app. When this is attempted the install will fail with the following message:
Why this change?
This change has been put in place to prevent any user or admin confusion between locally installed apps and apps installed from Atlassian Marketplace.
What's the alternative?
If you are an app developer, this may affect your development process. You will now need to use a separate instance to undertake testing with a local version of your app.