No, it does not update. You will have duplicate entries.
Also, if the brokerage firm issues corrected forms after you import, you will need to delete the original import and re-import the final/corrected forms. See How do I update or delete something I imported into TurboTax?
If you have already filed your return and the brokerage issues a corrected form you may need to amend your return. See Do I need to amend?
Related information: How do I import my 1099?
[Edited 3-11-2020|7:57 am PST]
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