Begin a thirty-minute Era and get a village to reach a prosperity of at least 400. Then, replace duplicate Natural Sources within the borders with different ones. It is accept for duplicates to exist before the end of the Era. However you must change them to unique natural sources or their upgrades (you can have only minerals as long as they do not repeat, and village prosperity drops below 400) before the era ends. You might need to open up some developments to unlock enough upgrade paths for your natural sources so they do not repeat.