Since 17w45a you cannot add a tag to an entity twice. Instead you get the error "Target either already has the tag or has too many tags".
However, when trying to add a tag for a group of entities of which some already have it shows it was added successfully for all of them.
Same for /tag <targets> remove respectively.
- Summon an armor stand with a tag
- Try to add the tag to the armor stand again
→ It fails as expected ("Target either already has the tag or has too many tags")
- Summon a second armor stand without the tag
- Try to add the tag to the armor stands
→ It says it added the tags to both entities ("Added tag 'test' to 2 entities")