Fix createRandomZombie function for DNA uniqueness

[xyz899]

Description

This pull request addresses the issue of duplicate DNA in the createRandomZombie function. When generating a random DNA for a new zombie, I noticed that the initial approach could lead to duplicate DNA values among the zombies. To ensure DNA uniqueness, I implemented a check that verifies if the generated DNA already exists in the zombies array.

Changes Made

• Added a DNA uniqueness check in the createRandomZombie function to prevent duplicates.
• If the initial randDna is not unique, re-generate a new randDna until uniqueness is achieved.

Testing

I have thoroughly tested the modified function with various names and verified that each new zombie created has a unique DNA.