Rats, my previous answer got eaten....
No you don't just double the sts, there's 4 sts on each side in garter stitch that you don't repeat across the row. Take off 8 from the 88 which gives you 80 then double that for 160 sts, and add the 8 sts back on. So you actually cast on 168 sts.
sue- knitting heretic