The Docker ecosystem already has CoreOS. Are Project Atomic's goals significantly different enough to justify a similar project?
You could put the same question: There is already Fedora distribution, does it make sense to start a different one?
I am not developer of Project Atomic, but from what I read, it will provide sligthly different features. The 10,000 ft view can be same, but even the fact you can run containers on Fedora (and RHEL/CentOS later) can be appealing to some users.
The biggest difference is SELinux support which is very important bit when it comes to security. SELinux plays very important role in OpenShift and this is same for Project Atomic. AFAIK CoreOS does not support SELinux currently.
And there are more differences, but I don't think it's important to name them (and I am sure folks will give you more answers). Having a choice is important. Actually it's the driver of Open Source.