lp duplex doesnt work

I tried double sided printing with

lp -o sides=two-sided-long-edge file.ps,

and

a2ps -s2 file.ps

on a duplex printer, but non of them works while

a2ps -=book file.ps,

prints a duble sided (but four pages in 1 ***) well!
any suggestions how to print a regiular double sided page?

Thanks
.

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