Hi Adrian,
With the stock CRM system you cannot duplicate an estimate or quote. However, this is something that can be done through customization.
What you would do is use the Insert Record Into Table action to insert a new Estimate, and use the values on the Estimate screen to input into the new Estimate record. The actions for this are similar to how the "Create Sales Order" button works on the Estimate screen. However, instead of creating a Sales Order, you are creating a new estimate.
Hope this helps,
- Adam