I know it is an old bug, but I get it as well, with the latest version. Using Redis storage. Mostly after a server timeout exception.
After that I get Key already exists exception when calling this function:
var crawlQueues = monitoringApi.Queues().Where(q => !q.Name.Equals("default", StringComparison.OrdinalIgnoreCase)).Select(q => q.Name);
I guess, the bug on the enqueued page is using somewhat similar code, except not filtering the default queue off course.