A: 'CREATE SEQUENCE emp_seq START WITH 1 INCREMENT BY 1 NOCACHE;' is correct for ensuring unique values without gaps as much as possible by not caching sequence numbers, which might otherwise be lost in a system crash.
B: CYCLE allows the sequence to restart when its max/min value is reached, which does not help with requirement 3 (duplicates are never permitted). Therefore, B is incorrect.
C: This lacks the necessary attributes like START WITH and INCREMENT BY which are crucial to defining a sequence. Thus, statement C is incorrect.
D: 'CREATE SEQUENCE emp_seq START WITH 1 CACHE;' might introduce gaps due to the caching of sequence numbers. This statement is somewhat contrary to requirement 5.
E: 'CREATE SEQUENCE emp_seq START WITH 1 INCREMENT BY 1 CACHE;' will provide continuous unique values, but may include gaps when the cache is lost due to a restart, yet it is more efficient and still generally aligns with the requirements. Hence, statement E is considered correct.
F: This lacks detail and is too ambiguous, lacking the necessary parameters. Therefore, F is incorrect.