The word is “cummerbund”. There is only one “b”, and that is at the beginning of the last syllable. Ergo, hence, and therefore, do not pronouce this word as “cumberbund”.
However, if your cummerbund’s design contains a pattern of whimsical cucumbers woven into the silk, you may call it a ‘cumberbund (note the apostrophe). But only once, in an attempt to make a lame joke.
Still, you should know that cummerbunds with whimsical designs are just horrifying. Unacceptable. The correct pronunciation for that sort of cummerbund is simple: “wrong”.
You should also know that any attempt to wear one in my presence will result in my absence.
Just so we’re clear on the issue.
Thanks for listening.