You have limited a published application to one instance per user. However when a user clicks on an application icon in StoreFront multiple times, it still launches multiple instances of the application.

Increase Multi-click duration in StoreFront
When using Citrix Workspace app in a browser, by default the UI blocks launching the same app multiple times within 3 seconds. You can increase the Multi-click duration timeout in StoreFront, see StoreFront documentation.

Problem Cause

Users are clicking the app icon again after 3 seconds, but before the initial instance of the app has completed launching. Instance is not yet registered with the delivery controller and therefore the delivery controller allows another instance of the app to be launched.