Powershell remoting and SPNs

I'm attempting to configure powershell remoting on a newly built server and am confused about something. Specifically, the WSMan listener is configured to use port 5985, but when I add the SPN http/computername:5985 computername$, it doesn't take, but when I add the SPN http/computername computername$, it does. How do I configure a server (e.g. a web server) that already has the http SPN assigned to a different account?