According to this answer,
"our seed is as the combined username and password that grants access to our bank account. If anyone obtains our seed, they can login and access our funds."
Despite some existing approaches to generate a safe seed, however consider I generate my seed "manually" that means selecting 81 characters between A to Z and number 9. And someone else also does the same. So, we will have same seed.
Firstly, is it possible such a situation? And if so, what happens?
You and the person who generates the other seed would see the same balance and both of you could spend the Iotas. The Tangle doesn't know how many people know your seed and would treat you like a single user.
Was already answered here:
Basically, there are 8.7×10¹¹⁵ possible seed combinations. This makes it very, very improbable, that two people ever generate the exact same seed.
That's why you do not create a seed manually. If you create it manually there is a good chance for a collision because human minds operate in similar ways. However, if you generate the seed randomly, the number of possible seeds is so high that this will practically never happen.
Who has the seed, has total control of the funds so if two people have the same seed, they both control the funds.
Another interesting feature is: Try to increment the first character of your seed. This will essentially just move you forward in your seeds address space.
So total seed space is not higher than 8.7×10¹¹⁵/27 removing 1st character
