Sorry about this. I have tested this and this is an issue with the system.
Currently what is happening is that the original bill entry has a transaction ID. Let's say 2100. This will make a pending asset based on that transaction.
Now, if you go to edit the bill with a Fixed Asset account code, it will change the transaction ID from 2100 to 2101. This new transaction ID is completely disregarding the pending asset from the 2100 ID.
In reality, Clear Books Accounts module should be making the pending asset only when the bill is created. This will prevent any duplicates to be made if you make an edit to a bill.
Please can you send us an email to contact support with this as a reference and the Support team will be able to pass this onto the developers to see if it can be fixed.