Double increase

Notice the place under the strand of yarn which runs between the stitch just worked and the next stitch

You are going to make one left first. Insert the left hand needle from front to back into the strand:

Knit this stitch through the back loop:

You will work again inside the same strand of yarn:

Insert again the needle but this time, making one right. Insert the left hand needle from back to front into the strand:

Knit this stitch through the front loop:

You have worked 2 increases in the same strand.