can I wipe my iMac twice?

I have an iMac 8,1 I'm passing on. I went thru the process of wiping it, then my son used it to upload some photos from a disk and send (via his gmail account in the browser) to his MacBook. nothing else was used just that operation. Anyways, I checked the Mac afterwards and noticed that my user name still appeared when I opened various apps. IS that normal after following all the stages recommended in wiping a Mac to pass on/sell? can I wipe it twice?