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I think I will abandon the idea of duplicate stitch on this. So, what will (hopefully) give me the curves? Well, plain old fashioned hand embroidery would do the trick, I'm sure, if I were any good at it. But I'm not. So I think I'm going to try surface or tambour crochet. I am more at home with a crochet hook, and I think that I'll be able to reproduce the curves I need as well as the thickness of the lines.
The other challenge - transferring this design to the knitted fabric. Here's my attempt:
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I'm basically using a piece of thin white yarn and a large needle to "trace" the design onto the sweater. Then I'll rip the paper off (gently, of course), and using the white yarn line as a guide, reproduce the design with yellow yarn in the surface crochet.
Well, that's the idea anyway. Stay tuned to see if it works or not!