I've checked both screenshots you've provided, Kiki2.

Let's start with the 1st one. I've replicated it and found out that it wasn't a duplicate entry. It was just one transaction. That's the reason why when you tried to delete the negative amount, it deleted the whole transaction. 

The report on your first screenshot is detailed. It looks like 2 transactions, but it's only one. Thus said, you can only add one class. I've attached a screenshot similar to yours. 

On the second screenshot, you're unable to change the account once it's connected to online banking/bank feeds. You'll have to disconnect it, change the account, then reconnect the bank feeds again. 

If you have further questions, we're just around. Have a nice day!