The already registered error means that the node conflicts with another node you have installed. You can find which by stopping node red and restarting in a command window and inspecting the startup log there.
If you want to remove it and @jbudd's suggestion does not help then you need to remove it manually. If you have a normal node red install then, in a command window, cd into your .node-red folder and run npm remove node-red-contrib-ical-events
It should say that it has removed nodes.
Then restart node red and refresh the browser page.
Do you have more than one ical node installed? The reason I ask is that node-red-contrib-ical-events nodes show a brownish yellow while node-red-contrib-ical-schedule nodes show as a light blue.
Do you have both installed? The easy to determine is to go to the hamburger menu (top right in editor) and select Manage palette then search for -ical- and see what you get back.