If you are using BB10 this an issue a lot of people have. Some have duplicate numbers, some get duplicate contacts. It usually happens with adding more then one email address.
When I restored my contacts from firechest backup to my wonderful new BlackBerry classic I found many numbers duplicated number of times..... I'm guessing I have to check the settings on firechest. I had this problem with the link when I went from 9810 to the Z10 using the Gmail contact backup.
Firechest has many issues. Once I did restore of SMS, it got mixed with contacts. Someone contact name and someone else SMS. BBLINK is only reliable at the moment for full backup and restore. But, be careful, sometime it doesn't work.
I've figured out that contacts is picking up a contact from everywhere it is listed. If I have a contact on Gmail and icloud the contact shows up twice. If I turn off icloud then the contact only shows up once. BlackBerry Contacts is not recognizing duplicats.
The contacts app is a huge pain for that. I can't figure out a foolproof way to eliminate duplicate entire. As a brighter way to look at it, it's definitely a much better error to have duplicate entries than to be missing some of your contacts.
If any really motivated developers out there are reading this, there is a lot of money to be made in a contacts organizer. I hope that eventually they will work this into Blend so that you can organize a master list of all contacts from all your accounts. There is currently an option in your contacts settings that says 'auto merge contacts' but it doesn't seem to be very effective.