decreases, be they single or double, can be part of the 'style' --one of the persnickity details of knitting.
this sampler(of decreases) has 2 examples of double decreases.
the upper one is Raised Centered decrease
(slip 2 together, (knit wise) k1, Pass the 2 slipped stitches over the knit one)
the lower one is the simpler slip 1, Ktog, pass slipped one over (the stitch that is the result of the knitting 2 tog)
both are neat and attactive.
the upper one is slightly more decorative.
decide which decrease you like, and stick with it!