Doppelganger is certainly a word one would not drop in casual conversation and to us it is a word that you want to have catalogued away so when you hear it you know the meaning and act informedly. But doppelganger is so obscure that one rarely hears it in use today. Doppelganger refers to an apparition or double of a living person. A double who looks like the President and can stand in for him when needed is a doppelganger.
The word doppelganger comes from German and means “double-walker,” and first appeared in fiction in the late 1700’s. The use of the word in a paranormal sense is recent and has no historical reference. We have heard all our lives that everyone has a double out there somewhere, we assume that for each the other is their doppelganger.
Undoubtably is around because of pronunciations and just plain ignorance. Many if not most dictionaries list it as a “non-standard” word, or something in common use that is not really a recognized word. If it were real, it would be an adverb meaning something that cannot be doubted, something without doubt. “The Wall Street Journal is undoubtably the top financial newspaper in the United States.”
What makes undoubtably so difficult is that it is not flagged by many spellcheckers, and even if it is they will identify it only as a non-standard word. In many cases the word “undoubtedly” is the intended word by the speaker or writer.