HTTP specification and browser implementations force a maximum number of concurrent connections to the same host (HTTP specification prescribes 2, browser implementations are usually looser). This means that letting a user open multiple instances of the same Lightstreamer application (that connects to the same LS host) will saturate the browser's connection pool, resulting in th eimpossibility to run the latest instance and blocking new subscriptions and unsubscriptions in the previous instances.

To prevent that, Lightstreamer implements a client-side mechanism that works in the following way:
If an application cannot connect to LS Server AND at least another instance of that same application is opened in the same browser space (another tab or another window that shares the same process), then a warning is raised to the user, explaining they should close one instance.

That warning is very important and cannot be disabled. Setting all the setDebugAlertsOnClientError(false) will avoid the duplication of the alert, but one alert, directed to the final user, will remain, because that's part of the framework logic.

But notice that you have control over the behavior of the creation of a new Engine when a similar one already exists. See the onSimilarEngineFound parameter in createEngine(). You can decide to prohibit any further instantiation of the same application or to share the same Engine aming multiple instances. In both the cases that warning would never appear.