While hitting tab, the focus steps to the next input field of the form, so I cannot reproduce selecting suggestions by using a tab. When I select a suggested tag, it gets stored only once.
However, if you found a bug within the autosuggest-jquery-plugin, can you inform its author Drew Wilson (http://code.drewwilson.com/entry/autosuggest-jquery-plugin), so that the code gets fixed for everyone in a new release and I will use the released version here?
I'll inform Drew, but I should probably duplicate with a bare version of autosuggest first. You should be able to duplicate the django implementation by hitting the down arrow once a tag is highlighted from autocomplete, then tab from there.