Debian :: Different Package Versions From Same Repository

I noticed today when I was offered by synaptic to update some packages that there seemed to be two versions available from the same repository (stable). How could that happen?

For instance if I check package "xscreensaver" installed version is 5.30-1+b1 and versions available are;
5.34-1 (testing)5.30-1+deb8u1(stable)5.30-1+b1 (stable)

Below is my /etc/apt/sources.list. I'm not sure why, but rows 1 and 6 are identical except for stable beeing replaced by jessie. Row 3 and 7 are almost same too. Because jessie=stable (at the moment), is there any point with having duplicated rows in sources.list?

Code:

Select alldeb http://ftp.se.debian.org/debian/ stable main non-free contrib
deb http://ftp.debian.org/debian/ stable-updates main non-free contrib
deb http://security.debian.org/ stable/updates main non-free contrib

deb http://ftp.se.debian.org/debian/ testing main non-free
deb http://ftp.se.debian.org/debian/ jessie contrib non-free main  <--- Same as first row?
deb http://security.debian.org/ jessie/updates contrib non-free main