If it is the first time you are making reference to someone/something, you have to use "as ~ as someone/something".

Example:

A: Why are you looking so down?

B: I can't help but feel that I am not as smart as my siblings lately.

However, if you were already in the middle of the conversation with someone and the reference point/noun as already been mentioned, then you can simply use one "as".

Example:

Coach: You may be new to the team but your skills is on par with many of your seniors.

Student: Thanks for the compliment but I am not as good (You may also choose to say "I am not as good as them", if you wish to).