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Someone told me that "goodbye" is only used when there is no chance of meeting again and "bye bye" means there are possibilities of meeting again.Is it correct?
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No, not in the UK - they mean the same thing. "Goodbye" is slightly more formal, and we would tend to say "Bye" rather than "Bye bye" informally. I would tend to use "Bye bye" if I'm talking to a child.

The person who told you this might have been thinking of French, where there is a difference between "Adieu" (when one is unlikely to meet again) and "Au revoir" (which literally means 'until we see each other again'). In classic literature, for example, in Shakespeare's plays, you might hear characters saying "Adieu" when they are leaving for good, but it is not in use in modern British English.
Thanks Anon