Tag duplication allowed in one Article

More then one same tag allowed in one Article. If user duplicates some tags in one Article, only one of each copy must be displayed.

Steps to reproduce:

1. Sign in in to antarctical and click "Write article"
2. Fill Article Title and Content with valid information
3. Enter 2 or more same tags, for example 1
4. Click "Create article"

AR: all tags added
ER: all duplicates added only once.

Note: There is a lot of related problems:

1. If user tries to delete one of duplicates, changes have no effect - in Article displayed the same amount of copies as in the moment of creation.
2. If user deletes all duplicates, changes apply, but if one of the deleted tags is put back again, there will be duplicates shown again, the same amount as in the moment of creation of the article
   Video: http://screencast.com/t/4vpfnIOYVy