This will add a task only if it doesn't already exist.
$ todo.sh addu "sell my bitcoins"
30 sell my bitcoins
TODO: 30 added.
$ todo.sh addu "sell my bitcoins"
30 sell my bitcoins
Already exists.
cd ~/.todo.actions.d && \
wget https://raw.githubusercontent.com/markcaudill/todo.txt-addu/master/addu && \
chmod +x addu