How to get a set response interactively

I tried 
?- setof(OS, mount(_, path([OS | _]), _W), Mounts).
_W = 2013-4-20 #60,
Mounts = [ostempserver1] ;
_W = 2013-4-22 #50,
Mounts = [ostempserver1] .

?- setof(OS, mount(_, path([OS | _]), _), Mounts).
Mounts = [ostempserver1] ;
Mounts = [ostempserver1] ;
Mounts = [ostempserver1] .

And some variations like
?- setof(OS, +Dev+W+Rest%mount(Dev, path([OS | Rest]), W), Mounts).
|    .
ERROR: Syntax error: Operator expected
ERROR: setof(OS, +Dev+W+Rest 
ERROR: ** here **
ERROR: . 

Is there a way I can get the set results without seeing all the
duplicate copies and having to step through them?

This is as close as I could get:
?- findall(OS, mount(_, path([OS | _]), _W), _Mounts), list_to_ord_set(_Mounts, Mounts).
_Mounts = [oscorn1, ostempserver1, ostempserver1, ostempserver1, oscorn1, oscorn1, oscorn1, oscorn1, oscorn1|...],
Mounts = [oscorn1, ostempserver1].

Thanks.
Ross Boylan