3.5mm to double RCA is probably a Stereo 3.5mm (two black stripes). You want a mono (one black stripe).
I think one of these will do:http://www.ramelectronics.net/html/R...if.html#rcabnc
There are probably less expensive cords available.
Or you could buy something like:http://www.maplin.co.uk/module.aspx?...29528&doy=18m5
And connect it to a regular RCA plug.
However, a stereo plug like you are describing might also work. If you already have one just try each RCA, as the digital signal will only go across one of them. The key part will be properly configuring the sound card/ALSA to output the right thing through 3.5mm output.