I looked back over my old comments and I only see mention of an exception. I definitely think it is the correct choice as it is exactly a value error, and continuing to execute could result in strange and hard to diagnose behaviour especially if the warning is missed it silenced. 



> On May 30, 2014, at 3:13, Tarun Gaba <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!")
> But as I recall, we discussed about issuing a warning rather than an Error, right?
> 
> —
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