This error pretty much annoyed the shit out of me where a temporary disconnect resulted in this error for a few minutes on freenode and irssi used to try using a different nick.
This can be solved by not authenticating using
/msg NickServ password mypass
If SASL is used for authentication, Freenode servers knows beforehand what my nick is and doesn't complain about this error. The how-to guide for it can be seen here: http://freenode.net/sasl/sasl-irssi.shtml
Another thing you might want to configure is: http://scripts.irssi.org/html/keepnick.pl.html
This is one of those issues for which I was lazy enough to not search about it.