Cannot add another version of content to project if one version already exists in project

Description

Steps to reproduce

This happens in both CMS and a Commerce sample site (QuickSilver).

In a CMS site

1) Create a project named Project_1.
2) Create a page named Page_1 with two different languages (for example, English and Svenska).
3) Go to Page_1 with the English version in Edit Mode.
4) In the Language Manager context menu, select the English version of Page_1.
5) Click "Add to translation project" then click OK. Now Project_1 has one item: Page_1.
6) Staying on the same page from the Language Manager context menu, select the Svenska version of Page_1 and add it to translation project.
7) Click OK. It is not added to Project_1.

Expected : Project_1 should have two items: English and Svenska versions of Page_1.

Actual: Project_1 still only has one item: The Svenska version.

In the QuickSilver sample commerce site :
If instead of adding the page to project, you add a Catalog Item, the same thing happens.