Dopplegangers do not play a significant role in witchcraft and to the best of my knowledge there are no traditions (certainly no British traditions) where they are heavily featured. As such there is no single interpretation and (as with anything) individual witches will form their own individual thoughts and opinions as to whether there is any credibility in the concept.
A witch will however be open to the idea that a spirit can leave the mortal home of the body. A doppleganger may be interpreted as a meeting on the astral (one soul seeking out its double via the astral plane) or even the soul meeting itself at a different stage of its life. If this is the case it makes sense for the soul to appear in the same physical form as the individual meeting it. Witches who believe in reincarnation may interpret the doppleganger as a visitation from a previous (or future) life and one possibly with a lesson to teach. Such an interpretation doesn't fit with the legend of ill luck - if anything such an interpretation would be a positive one suggesting new knowledge and wisdom would be forthcoming.
However a different view might be that the doppleganger is not so much an omen of our own death but the means by which we embrace death. If we see the doppleganger as our guide to the otherworld then it isn't too difficult to see how the associations with death have come about. Perhaps the best guide into death would be our own self. What could be more safe or familiar?
The doppleganger legend varies as to whether it is defined as a physical living person or as a non corporeal entity. The latter would be easier to reconcile with the spirit world of the witches. With regard to the doppleganger as a real life living duplicate person my own view is that this interpretation leans too heavily on the idea that an identical mortal home must house an identical spirit or soul. There are logically only so many ways you can arrange two eyes a nose and a mouth without inevitably incurring some duplication. I'm inclined to think physical similarities even to the point of mirror image are just coincidence. There are after all seven billion people on the planet - you'd be pretty hard pressed not to get a few duplicates.