If I understood your question correctly, Tracebility matrix 
is the solution.

Tracebility matrix maps the test cases with the 
requirements so that no requirement is missed out.

In case of any new requirement, add the new requirement to 
the Tracebility matrix and write a new test case thats maps 
the new requirement.

Well managed Tracebility matrix will let you know what test 
cases needs to be tested as per the features added or new 
requirement.

Regards,
Imthiyaz. Shaik

I got your critical question and valid real time one .
Sol : suppose you write the test cases for the version 6 
then few days after need to write the new fuctionality 6.1 
in such case you should tagged the new version tcs while 
you were working on excel and check  that duplicates will 
not occur and after few days you get major related new 
functionality name 6.2 .
In such case simply keep all the tcs of the previous one 
and new version tcs with tags ( this will helpful even 
while in track ) .
Here major understanding is when ever any modification 
happen on requirements Duplicates will not follow and new 
version commits the old version Major functionality .

Traceability matrix is fine but when a new defect happen we 
can identify the duplicates .
If you want try a simple  trial major scenario .
As a tester should be ready to run in all the 
circumstances .

Any concerns /help koteswara.9.rao@gmail.com