The bug

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.

How to reproduce

1. Summon an armor stand with a tag

   /summon armor_stand ~ ~ ~ {Tags:["test"]}
   

2. Try to add the tag to the armor stand again

   /tag @e[type=armor_stand] add test
   

   → It fails as expected ("Target either already has the tag or has too many tags")

3. Summon a second armor stand without the tag

   /summon armor_stand
   

4. Try to add the tag to the armor stands

   /tag @e[type=armor_stand] add test
   

   → It says it added the tags to both entities ("Added tag 'test' to 2 entities")