You’ll likely see this abbreviated DYO (double yarn over), but this doesn’t seem to be as common as some other knitting abbreviations so I’ve seen it other ways too (e.g., double YO).
Regardless of the way it’s abbreviated, you work it the same way.
How to work a DYO
Step 1: Work up to the DYO
Just work whatever stitches are specified. You don’t need to do anything unusual or different.
Step 2: Work the DYO
It’s just like a yarn over, but you’re wrapping the RN TWICE with the yarn.
Step 3: Work the rest of the row as specified.
Again, nothing unusual here. The different part happens on the next row.
Step 4 (WS): Work up to the DYO again.
Step 5 (WS): In general, you’ll either knit and purl the DYO or purl, then knit it.
Your pattern should specify which. Honestly though, it doesn’t make a big difference,
Here’s a DYO worked with a pk.
And here’s a DYO compared to a regular YO again. The DYO is much larger, and that’s often the reason that designers use that stitch.