The problem is both solutions are inadequate in some cases. I don't want to belabor this though. Warning is fine we can do some better checking later if we need to. 



> On May 30, 2014, at 8:04, Damián Avila <notifications@github.com> wrote:
> 
> In bokeh/plotting_helpers.py:
> 
> > @@ -336,6 +349,13 @@ def _new_xy_plot(x_range=None, y_range=None, plot_width=None, plot_height=None,
> >  
> >          tool_objs.append(tool_obj)
> >  
> > +        #Checking for repeated tools
> > +        _tools_in_list = [i.__class__.__name__ for i in tool_objs]
> > +        repeat_tool = [_tools_in_list.count(i)>1 for i in _tools_in_list]
> > +
> > +        if any(repeat_tool):
> > +            warnings.warn("one or some tools are being repeated!")
> It should not be an Error if a user wants to include both PanX and PanY as separate tools in the same plot?
> 
> Interesting question, but not necessarily it is an error... maybe a user wants to have specifically panx and panys tools, because it makes easier to interpret his/her plot...
> 
> Ah And Damian mentioned about "throwing a warning".
> 
> Yes, I see "throwing a warning", because I thought it would be enough, but reading the @bryevdv answer, I think an exception is better...
> 
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