Recently I setup a crypto wallet which used a 12-word BIP39 passphrase as security. I noticed something peculiar when I went to setup an additional wallet. The first and last words of the previous wallet I created were exactly the same on the new one. I thought this was odd so I created many more wallets to try and determine if that behavior was just a fluke.
On the next dozen or so wallets I didn’t get the same first and last words again, although I did on occasion get one or the other. Next I took the original first and last word then input words randomly that were present in my other created wallets in slots 2-11 to see if the wallet would be valid; they were valid 100% of the time. For the last test I decided to take the first word of my original wallet and repeat it twelve times, which also produced a valid wallet.
Is this typical behavior BIP39 mnemonic passphrase cryptography? If not what would the cause be other than a botched implementation?