If I manually entered bank data and later import data from their site, does that creat a double entry of some data or does it just updates everything?

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?

• If you need to amend your tax return, see How to amend (change or correct) a return you already filed. 

Related information: How do I import my 1099?

[Edited 3-11-2020|7:57 am PST]