In the top-right corner of your chart, click → Save as copy. In the window that opens, enter a name for the new chart and click Save.
At the top of the chart, click → Duplicate. In the window that opens, enter a name for the new chart and click Apply.
Go to the workbook page with the chart. In the row with the chart, click → Duplicate. In the window that opens, enter a name for the new chart and click Apply.