Multiple instances of OnConsoleStatusListener are unnecessary and would produce confusing output (from the print-outs of each instance), so IMO logback should prevent this from happening even if the user specifies more than one <statusListener> tags with OnConsoleStatusListener or simultaneously sets the debug flag. While it might be obvious not to use both options at the same time, there may be use-cases where the presence of either option is unknown ahead of time. For example, an application's logback config, with its debug flag set to true, pulls in a remote configuration from a URL (via <include>) that may have its own <statusListener class="OnConsoleStatusListener">.

The code to prevent this seems pretty simple and low-risk to me, but you probably have more insight into this than I.