Are SqlServer Sequences, guaranteed to return different values for concurrent transactions?
Hence, in my case, there is no duplicate checking while inserting.


If you really do not intend to allow duplicate values, you should create a UNIQUE constraint on the column where the ciphered text is to be stored.  Your application will have to be coded to retry on inserts that fail with a unique constraint error.


  The thing I need to know is that whether it is probable for two clients calling NEXT VALUE FOR function nearly at the same time, and get the same values?


While it's unlikely, under normal circumstances, to get the same value on two separate calls to NEXT VALUE FOR from the same sequence, there are things that can cause it to happen:


the sequence was specified with the CYCLE option, and the maximum value has been reached (causing it to roll over)
the sequence was reset (ALTER SEQUENCE RESTART)
as documented, accessing the same sequence object multiple times in the same statement will produce the same value each time, so make sure not to do something like this expecting two different numbers:


SELECT 
    NEXT VALUE FOR dbo.TestSequence,
    NEXT VALUE FOR dbo.TestSequence;


You may also be interested in reading Sequence Tables, by Paul White.