GNU cash - will automatically duplicate transaction

You are incorrectly performing single entry book keeping.

Within the Sales account, you are pairing it with Sales account.

You should type Bank or Debtor on the right hand side of the entry.

GnuCash is a double-entry accounting package, so every transaction needs to have 2 accounts: one account is debited and the other account is credited.

For example, when you buy something, you will pay money from your Bank account and increase the value of your Assets by the same amount.

In practical terms, what you would normally do is open up a window to one of the accounts and enter the other account in the "Transfer" column of a transaction.

What you have done is to open up the "Sales" account and put the same account into the "Transfer" column. This takes money out of the "Sales" account and puts it right back into the "Sales" account, hence the two lines that appear.

If this is a simple cash transaction where you sell something and put the money straight into a bank account, it would be more common to open up the Bank account window, enter the income amount, and put "Sales" into the "Transfer" column. This would increase your Bank account balance (treated as DR) and increase your Sales account balance (treated as CR). If you then open your Sales account window, you should see the matching entry there. It's still 2 entries, but one is in the Bank account and the other is in the Sales account, instead of both being in the Sales account as you currently have it.

If you have more complicated sales transactions such as credit terms, ask your accountant about setting up an Accounts Receivable or other relevant set of accounts. The principle is still the same, but the way you structure your accounts will need to be tailored to your specific requirements.