I'd already installed after effects cs6. I want to use CC(2017), Does it matter if I intalled it in duplicate?
In duplicate usually means trying install the same version twice.
In your case, you want to install multiple versions which is no problem at all. Go ahead. Different versions all install into separate folders on your hard drive. Just avoid running them at the same time.
Many Cloud members have several different versions of the software installed due to plugin compatibilities or collaborative workflow issues.