When a wallet is duplicated and you add new receiving addresses to one copy then the other copy will secretly also have those addresses. However, the other copy does not show the addresses under the Receive tab even if those addresses have already received some funds. I don’t think it’s a bug but it can cause some confusion (happened to me).
I edited the first post in this topic. Originally, it was somewhat a false alert.
If you add new receiving addresses to the copy you haven’t yet added new addresses to, they will show in the receiving tab.
This behavior is due to the key pool, a property inherited from Bitcoin. By default, there are 100 keys stored in a wallet’s key pool. They aren’t displayed in the receiving tab until you click New Address. What actually happens when you click New Address is the oldest address in the key pool is added and a new key pair is generated and added to the key pool.
The key pool is designed to allow you to backup your wallet, then add a new address while ensuring the new address you added after you backed up is actually in your backup.