every testcase has its own version no ,with the help of 
that we can write unreaptable tcs

By using traceability matrix .

the test cases are mapping with the requiremnet

wonderful question Hivna. Traceability matrix will be
correct only when one test case is written for each
requirement. But it wouldn't be the case always. 
When many testers are writing the cases for an
requirement/module, it is the job of the team lead(who
approves the testcases) to check for duplicate test cases
and parting the job properly so as to avoid duplicacy.