This BackgroundWorker is currently busy and cannot run multiple tasks concurrently

Question

Simple: Don’t start the BackgroundWorker twice.

You can check if it is already running by using the IsBusy property, so just change this code:

worker.RunWorkerAsync();

to this:

if( !worker.IsBusy )
    worker.RunWorkerAsync();
else
    MessageBox.Show("Can't run the worker twice!");

Update:

If you do actually need to launch multiple background tasks at the same time, you can simply create multiple BackgroundWorker objects