Mine is.
I've seen a number of people with the pre-title of "sir", but never anyone with "dooble" or any edits of that, actually.
The only time I've ever gone to use this name to register on something and it's taken, is due to the fact that I allready registered there and forgot my acount details. .-.
Although, the closest to "sir dooble" I've seen, is "sir elbood" which is dooble backwards. Mind you, this was my brothers chosen name for a number of things (mainly club penguin at the time). So, since he got that name from mine, I guess they both still stand as original.
Also, personally, I find that people with 1-3 numbers add them for the extra effect, not necesarilly to continue to have their name if it's taken. Names with lots of numbers are either made at a young age or in a lapse of common sense, I guess, or they're made in order to get past an allready taken name.
I do find that name's that are like "xX[Name here]Xx" are usually used as for taken names. As far as I'm aware, that is.