Photoshop problem:cannot select multiple layers

hi, there, i hope anyone who know this could help me out,i am frustrating T_T
is like this: i couldnt select multiple layers in my photoshop,
why is it?
i tried:
cmd+shift+click
ctrl+shift+click
shift+click
shift+alt+click

but all cannot,i sometimes have multiple layers, and then i want to duplicate all of them,
so it is troublesome i duplicate one by one, but when i want to move them around,it is okay because i can link them. but i cant select all of them.
if anyone know this,please help me.
i am using macintosh.

thanks in advance.:smiley: