Back up first
Create a company file backup before importing a new bank file. This gives you a clear recovery point if the period is duplicated.
Import safety
Avoid importing the same bank period twice.
A converted file cannot know the full state of your company file. Use backups and review transaction IDs before a second import.
Keep the source period visible
Note the start and end dates of the file. Do not combine overlapping bank exports unless you have checked the overlap.
Treat duplicates as warnings
The same date, description, and amount can be a legitimate repeated charge. A converter may flag a possible duplicate, but it should not delete it automatically.
Test repeated imports
For a new QBO workflow, import the same test file twice in a blank company and record what QuickBooks does before using a live file.