If you are running tmux locally and you shell into another machine to access tmux remotely, you will suddenly find yourself in tmux inception. You will have a tmux instance running within your local tmux instance. If you have the same prefix key set for both, then you may be wondering how you can send a tmux command to the inner tmux instance.
If you press your prefix twice (e.g. <C-a> <C-a>), then the second prefix
will be sent to the inner tmux instance which will then be listening for
the rest of your command. So, to open a new window within the inner tmux
instance, you can hit <C-a> <C-a> c.