Hey. It seems like you already have testrpc running. Check it using the ps command:
$ps
PID TTY TIME CMD
4195 pts/0 00:00:00 bash
5551 pts/0 00:00:01 node
5627 pts/0 00:00:00 ps
Here, the process node (5551) is the testrpc. Kill it using this command and you will be able to start testrpc just fine:
kill -9 <pid>
In my case, it is
kill -9 5551