Created attachment 107754 [details] xmi file with deleted and therefore unreachable entities inside Umbrello version 2.23.1 for Windows (Windows 8) When you create an entity and then deletes it, then there is no way you can create another entity with the same name as the deleted one. It shows the error message: "That name is already being used". Inside the xmi structure it saves the deleted entities, but there is no way to tell Umbrello to remove them completely. I guess this bug is related to the way Umbrello handles the UNDO-chain. In the attachment the entities "Lista2", "NuevaLista" and "MapaProducto" are deleted entities that still exist but can not be reached within Umbrello.
From your report I assume that 1. you added an entity to the diagram plane and deleted it from there and 2. you do not have the tree view visible (click on settings->tree view) Removing entities from a diagram only removes it from the diagram not from the list of available UML objects. Available UML objects are displayed in the tree view. After opening the appended file the mentioned objects are displayed in the tree view. To add an available entity to a diagram you need to drag it from the tree view. If you want to let umbrello deal with already available UML objects by choosing "New Entity" feel free to open a new feature request.