I am working on a mitten design using three colours to form a geometric pattern and part of it really looks best with the corners of some diamonds in a contrast colour but that would result in an entire round with just two stitches in that contrast colour. I decided I had three choices. First was to carry the third colour right round, work those two stitches, and twist the yarn periodically on the back of the work. Possible but awkward. Second was to ignore that third colour and instead work those two stitches in one of the other two colours. Simpler but less aesthetically pleasing and not the look I wanted for that part of the design. Third was the option I chose - duplicate stitch or Swiss darning, which involves using a darning needle and a length of yarn in the colour you want to sew a stitch over the knitted one and make it appear that the stitch is knotted in a different colour.
Here is the mitten with the "offending" stitches - the blue ones at the top and bottom of the central diamond.
Here you can see the yarn coming forward again at the top of the second leg of the left-hand stitch and the needle entering at the point of the V again to complete the second leg of that stitch.
After that, weave the yarn through the back of the knitted work to re-emerge at the top of the lower pair of blue stitches. Duplicate stitch over them in the same way. In this picture, the right-hand leg of the right-hand stitch has been stitched over (still following?!) and the yarn and needle are in position to stitch over the left-hand leg of that V.
With careful sewing, the colourwork pattern is complete. Here is the final result (and compare the duplicate stitched centre diamond with the two fully-knitted ones on either side). If you didn't know, you'd have to look very closely to spot the difference!