You can check whether or not a link has been submitted by plugging it into the searchbox. If it has been submitted, it will come up with a message saying "This has already been submitted:", followed by a link to the submission. Reddit does the same thing. Plugging a link into the Reddit search box will show you when a link has been submitted, and to what subreddits. To access a list of users and hashtags being followed, click on your username on the left. You can then see the number of followed users and the number of followed hashtags. Click on the number to see what the users/hashtags are. Looks like you already found the answer to your last question.
Also how do I add a hashtag to follow? Previously I just went to community under the discover tab, and picked a few. Say I wanted to follow a #firefly, when I search firefly under the search bar no such hashtag comes up. Or is this just such a small community right now that a lot of hashtags have never been created, since they've never had a post? EDIT: sorry for so many questions. One more: I'm getting comment and post replies via email. Can I choose to get this through the mail under "my hubski"? What usually goes into mail under "my hubski"?
You must understand, hashtags isn't like subreddits on reddit. They're more like Hashtags on twitter. When you post something, you choose a tag for it. This just means that the people who are going to see your link are: Those who follow you. Those who follow the tag you've chosen. And those who follow those who share your post.
Tagging is for choosing what demographic of Hubski you think would find your post most interesting. So when you find something that is related to Firefly, you'd tag it as Firefly, this informes all that follow you that you're interested in Firefly, and if they agree with you in liking Firefly, they might just choose to follow the tag aswell. When it isn't there when you search for it, it's because noone has tagged something #Firefly. This isn't necessarily connected to this being a small community, but might as well be because other people interested in Firefly have simply chosen to use the tag #Sciencefiction, because it may resonate with more people, and it has a guarenteed amount of people that will see it. You can always create the tag #Firefly if you wish to, by posting something and tagging it as such.