Negation of the tag selector not working correctly

I have a creative world with one armor stand in it. I execute the following command two times from a command block:

/scoreboard players tag @e[type=ArmorStand,tag!=test] add test

And the successful execution of the command is written two times in the chat log. I thought after the first execution it should no longer be successful, because the armor stand now has the tag and the selector shouldn't find it anymore.