try adding setopt HIST_SAVE_NO_DUPS to your .zshrc

you could also have a look at HIST_FIND_NO_DUPS and HIST_IGNORE_ALL_DUPS (more options at http://zsh.sourceforge.net/Doc/Release/Options.html )

If you still use bash, now is a good time to upgrade to zsh

'Upgrade' and a sourceforge link don't usually go together :-/

I'm a zsh user but am surprised that you guys are still using sourceforge instead of github.