When adding a property to Wikidata items using the web interface, it is easy to unintentionally create a duplicate property entry - particularly on large items, where you may not notice that the property already existed.
This isn't a major problem - many properties are intended to have multiple values, after all! - but can be a little frustrating:
a) it can be confusing for the user, as after creating the new Pxxx line it "snaps up" to display alongside the existing Pxxx line, and so may seem like it's not been added when they look at the end of the page;
b) it means someone has to do a little work in future removing the duplicate.
A way round this might be to highlight in some way that a property Pxxx already exists. For example, when a user goes to add a new property to an item, and selects a property, the interface could check if it exists. If so, there's various ways to highlight that this is a possible duplicate:
* the blue outline box could turn to orange;
* a note could appear on the left underneath the property name: "An entry for P569 (Date of birth) already exists for this item".
* the existing lines could "snap down" to be displayed alongside the new line for that item