www.alphadictionary.com

This word is without any doubt a negative orphan. Or is it? Even though its adverse exists, I, without a doubt, have never heard it said. It may be dubitable but I maybe heard dubitable.

It began life as an orphan, but got an offspring. Etymonline says indubitable is from the mid-15th century, and dictionary.com gives a first quarter 17th century origin for dubitable. I'm no professional, but I'd say that makes it a back formation.

It is undoubtedly from the same source as undoubtable!