Try to destroy the widget before creating a new one:
var stackedAreaLegend = dijit.byId('legend1');
if (stackedAreaLegend) {
stackedAreaLegend.destroyRecursive(true);
}
stackedAreaLegend = new dojox.charting.widget.SelectableLegend({
chart: chart1
}, "legend1");
stackedAreaLegend.refresh();
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