docs(README): avoid double-source in example .bashrc

Co-authored-by: Koichi Murase <[email protected]>

Author: Rudxain

README.md

@@ -19,11 +19,13 @@ list of operating system distributions, package names, and available versions.
 Depending on the package, you may still
 need to source it from either `/etc/bashrc` or `~/.bashrc` (or any
 other file sourcing those). If you have _only_ bash >= 4.2 installed, you can
-do this by simply using:
+do this by using:
 
 ```bash
-# Use bash-completion, if available
-[[ $PS1 && -f /usr/share/bash-completion/bash_completion ]] && \
+# Use bash-completion, if available, and avoid double-sourcing
+[[ $PS1 &&
+  ! ${BASH_COMPLETION_VERSINFO:-} &&
+  -f /usr/share/bash-completion/bash_completion ]] &&
     . /usr/share/bash-completion/bash_completion
 ```