Prevent Duplicates
You could make a Composite Key of fields that are likely not to duplicate
Date / Amount / Description?
Unless you buy the exact same thing twice. Or the Date has a time too

Almost all bank downloads that I have seen have a transaction id from the bank in it somewhere even though it may not be apparent.
For sure all that I've seen have a transaction date/time stamp that you could use in determining uniqueness of the record.  Once you have that figured out you use that to not duplicate import data.

If you could copy a small sample and post it here, or take a screenshot it would be easier for us to help you.
Of course change or redact personal information as needed.

Below is a sample format CSV File from the bank.  I thought about using a Composite Key, but I would have to almost use the entire columns so I am not sure if that would be a good idea.

"Date","Bank RTN","Account Number","Transaction Type","Description","Debit","Credit","Check Number","Account Running Balance"
1/5/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"POS","DDA PURCHASE *9717 403482     LOWE S  2551","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"DIRECTDEBIT","TD BANK          MTG PYMT","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"DIRECTDEBIT","EVERSOURCE       WEB_PAY","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"DEBIT","VISA DDA PUR 469216     SXM SIRIUSXM COM ACCT","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"POS","DDA PURCHASE *9717 318219     STATELINE PET SU 137 PLA","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"DEBIT","VISA DDA PUR 469216     SXM SIRIUSXM COM ACCT","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"DEBIT","VISA DDA PUR 469216     WALMART COM AY","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"POS","DDA PURCHASE *8473 08954311   MARKET BASKET","000.00",,,0.00
1/4/2021 0:00:00,1zzzzzz4,6yyyyyyy6,"POS","DDA PURCHASE *8473 35350043   WAL MART SUPER CENTER","000.00",,,0.00

I trust this is helpful

EDIT by Scott Axton - Removed Banking information to protect account.

Do any of the dates contain a time element or are they really all 0:00:00 ?
If the date contains time that would be the best bet for uniqueness IN COMBINATION with the amount.
You are going to probably have to use a Composite Key described in the Prevent Duplicates video Alex suggested.

Another option - does your bank provide other methods of download such as Quicken/Quickbooks, XML, other?
Those formats if available may offer more detail than the CSV format.

The bank does offer QFX format for downloading transaction.  I have not been able to find a way to download QFX into Access.  I was thinking about using a primary key when I download the transaction.  Then run a query to put the Index into a field on the Register Table and use that Field as a composite key.  I haven't tried that yet, but I will do so and advise the results.