Prevent duplicate links from being added to a group

[tommcclean]

Describe the bug
Presently when creating a group of links on the website, a user can enter the same link multiple times.

To Reproduce

1. Go to https://cmpct.io
2. Click on the button to start a link group
3. Add the same link more than once

Expected behavior
A message should be displayed telling the user that the link they tried to add has already been added, a duplicate link should not be added to the group

Additional context
As the site is localised, any new messages will need to be specified in the <i18n> block at the bottom of the single file component.

[tommcclean]

Developed by adding a distinct in the API layer.