Both dramas start with two classmates falling in love and one of them tragically dying and mysteriously back to life, making them experience what could have been and ultimately coming to terms with everything and being able to let go.

Differences:
Eien no Kinou is a BL.
In Boku Dake ga 17sai no Sekai de, there is a time skip of several years, before the decaesed comes back.

In both dramas a group of people find themselves at a strange place without any way to contact their home or families. They are trying to survive and find a way back while forming their own social dynamics within their group.