async-shell-command-buffer is a variable defined in `simple.el'.
Its value is

What to do when the output buffer is used by another shell command.
This option specifies how to resolve the conflict where a new command
wants to direct its output to the buffer `*Async Shell Command*',
but this buffer is already taken by another running shell command.

The value `confirm-kill-process' is used to ask for confirmation before
killing the already running process and running a new process
in the same buffer, `confirm-new-buffer' for confirmation before running
the command in a new buffer with a name other than the default buffer name,
`new-buffer' for doing the same without confirmation,
`confirm-rename-buffer' for confirmation before renaming the existing
output buffer and running a new command in the default buffer,
`rename-buffer' for doing the same without confirmation.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.