Switching between Prewiev and Write mode: duplicated tag's should be deleted

Switching between Preview and Write mode: duplicated tag should be deleted

Pre-condition:

1. User is logged in
2. Create Article page is opened

Steps to reproduce:

1. Input 2 or more identical tags, for example: 1, 1, 1
2. Switch to Preview tab
3. Switch to Write tab

Actual result: tag line consists of all tags from step 1
Expected result: all duplicates are deleted

The same behavior when user tris to edit Article