Hmmm, it doesn't seem like it would be that rare that the same key is produced. Very interesting. Thanks for replying and explaining it.
The current (english) wordlist for BIP39 has 2048 words in it... a "typical" seed uses 12 words... some wallets use 24 words... even with just 12 words, the basic calculation for the number of combinations is:
2048
12 = 5.4445179 * 10^39
With 24 words that would become:
2048
24 = 2.9642775 * 10^79
even discounting "invalid" seeds (the last word is actually a checksum, so not ALL combinations of words actually generate a valid seed mnemonic) , that's still some fairly large numbers
Then you can factor in the fact that you can also add a "passphrase" onto the 24 word seeds... which pretty much makes the number of combinations infinite.