Due Diligence for each Purchase Order or a whole Contract?

A company in the Netherlands (operator) buys wooden products from a supplier in China. They sign a contract for 1000 pieces, but instead of receiving everything at once, they place multiple purchase orders over time.
Under the EU Deforestation Regulation (EUDR), do they need to do a due diligence check for each purchase order, or is one check enough for the whole contract, as long as all 1000 pieces come from the same land? What would happen if they are coming from different pieces of land?