In the context of a new project, it can be useful to start from a previous proposal to save time. The "Duplicate" feature allows to create a copy of a proposal and to replace the customer and the project context.
If you are trying to create a new proposal version (same customer and same project), it is preferable to use the "New version" feature. Read help article about the "New version" feature.
The duplication feature is available from the advanced options of a proposal, as shown with the screenshot below.
As soon as you have clicked the "Duplicate" option, you will be asked to select a new customer before accessing the newly duplicated proposal.