When adding an article to a proposal, order or invoice via the search field and the search criteria meets exact the article → so the article number is filled in automatically the article cannot be added.
When using a less exact search criteria and the dropdownlist appears, it is possible to choose and add the same article.
This is the errormessage which appears
The selling price can’t be lower than minimum allowed for this product (159 € without tax). This message can also appears if you type a too important discount.