This is usually due to importing multiple files individually but the File selection not being cleared after each import, or the Import button having been pressed multiple times for another reason.
To Re-Import the bill files correctly:
Delete the Bill/Sub Bill Heading from Estimating (this will delete all the bill items within)
Recreate the Bill/Sub Bill Heading
Reimport the File
Remember to clear the file name out from the bill section you have imported before loading and importing the next one
If importing multiple files at the same time only press the Import button once.
Items should have imported only once per bill section.
If not check the formatting of the file is correct and retry.