There is a setting in QuickBooks Online that you can turn on or off, that will warn you if a duplicate bill number is used (see screenshot below).
This can be found in your QuickBooks Online file > Settings > Account and Company Settings > Advanced > Other preferences
Click on Settings in the top right corner > select Account and Settings > Advanced (on left side) > scroll to the bottom to see 'Other Preferences' > Click on 'Off' beside 'Warn if duplicate bill number is used' > then put a tick in the box beside it.
When this setting is turned off, QuickBooks Online will allow you to export bills with the same bill number so you may end up with duplicates in QuickBooks Online.