I'm sorry for the frustration. If you let me know what username you're trying to sign up for, I can double-check in our system to see if there's already a WordPress.com account with that username.
As long as the username isn't being used already, you'll be able to sign up for a new account with it. It sounds like your browser is giving you fits (logging you back into your account when you want to be logged out). Here are a couple ways you could work around that:
1. You can use a different browser to sign up for your new account. You can download the latest versions of popular browsers here: http://browsehappy.com/
After you open the new browser, you can go here to sign up for the new account: https://signup.wordpress.com/signup/
2. You can use Safari's private browsing mode (by going to Safari > Private Browsing in the Safari menu). That will make sure you're not logged in to your contrast10 account while you try to sign up for your new account.
If you keep running into problems with this, can you please make a screenshot of what you see on the signup page and on the next page after you fill out the form there? You can upload screenshots to your Media Library under Media > Add New in your dashboard, or you can use an online service like Snaggy.