Can I do a backup without extension functions?

I use the PostGIS extension on my database. The thing is when I do a backup and then a restore, all the PostGIS functions give a warning during the restore process, because the functions already exist, as the extension is already installed. I have to scroll through all the warning to make sure there isn't a real error. So what is the right way to backup / restore, so I can have a clean restore process?