For categories, if I watch one, and then another, I now am watching the union of those two categories. I think it should work the same way for tags. So, for instance, in the example in the OP, I should now be watching both the foo and bar tags.
In addition to doing this for the sake of consistency with the UX for categories, consider this case:
User A uses some combination of slack commands to watch 7 particular tags in a given channel.
A month later, User B thinks the channel should also watch some 8th tag.
If they just do /discourse watch tag:number8, it’d be unfortunate if that silently destroyed the work of User A from a month before.
…
There should probably be a command to un(watch|follow|mute) tags and categories explicitly instead if you want to stop watching any of those first 7 tags.