How can you remove duplicates from bash history?

It is possible to remove duplicated lines which are already in .bash_history by running

nl ~/.bash_history | sort -k 2  -k 1,1nr| uniq -f 1 | sort -n | cut -f 2 > unduped_history

followed by

cp unduped_history ~/.bash_history

I would also recommend to put the following in your ~/.bashrc:

export HISTCONTROL=ignoreboth:erasedups