Hi leokong18,

Thanks for getting back to me here.  When you download the file, you can remove any transactions prior to uploading it into QuickBooks Online.  This will prevent duplicate entries.  However, you'll also need to remove any blank lines or columns from the file, to ensure a smooth transition.  Once you've made the necessary changes to the file, select 'save as' and give it another name.  This will ensure your original document is also saved, should you need to refer back to it.

Be sure to let us know if this helps.  We are here for you!