I have a question regarding the word ana (alif noon alif). I understand that the second alif is never read when continuing.

Question
Assalaamu alaykum

I have a question regarding the word ana (alif noon alif). I understand that the second alif is never read when continuing. One sheikh said that it is because the prohibition of two alifs both sides of a letter. But I am confused there are many words that have that oblong zero in them and sometimes there is only one alif in it and other letters which have alifs both sides that are read. Please can you clarify this for me.

Jazak Allahu Khayran for all your great help

Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh. 
The second alif in the word is not pronounced by Hafs when when continuing but is read when stopping. This does not hold true for all the qira'aat.  Some of the qira'aat read the second alif when continuing if there is a hamzah qat3 at the beginning of the following word. 
There are seven different words in the Qur'an which have the same ruling (in most cases), and there is no relationship between the lack of pronunciation of the alif and the letter after it. The symbol which looks like a zero over the alif gives us an indication that is is established when stopping but not when continuing.
 
The seven alifaat () and more details can be found at:http://www.abouttajweed.com/stopping-on-the-ends-of-words/stopping-on-the-ends-of-words-with-weak-endi-3.html
In all the seven alifaat found detailed in the above link, there are some of the qira'aat which pronounce the alif when continuing as well when stopping. 
Wa iyyaakum