Really, few billion years? I didn't know the exact calculation I just remind that it will take atleast 100+ years to brute force written by someone from bitcointalk in the technical discussion thread and now its more interesting. 
You can do the math yourself without too much trouble. For a 128 bit seed phrase, then there are obviously 2128 possibilities. Divide that number by 1012 (which is 1 trillion), and that's how many seconds it would take to generate all possible seed phrases if you could generate 1 trillion a second. Divide that by (60*60*24*365) to turn seconds in to years. Here it is written out: https://www.wolframalpha.com/input?i=%282%5E128%29%2F%2810%5E12%29%2F%2860*60*24*365%29But I also ask if the blockchain doesn't have an algorithm to immediately detect an attempt of duplicate upon generating a coincidental address.
No, it doesn't, and indeed, how could it? When you generate an address using any offline wallet, how could the network know that you have generated that address? And if you do import a seed phrase or private key and recover addresses which already have funds on them, then how could the network know if you had just generated them by chance or you were simply restoring a back up? You would need to create a central database of addresses linked to proof of identity, which obviously violates many of the core tenets of bitcoin. Instead, addresses are kept safe due to the sheer size of the math we are dealing with. (It's also worth noting here that the number of possible private keys and possible addresses absolutely dwarfs other things we rely on many possible combinations for, such as all your passwords, credit card numbers, SSNs, the radio frequency of your car keys, for example).