May I know the rulings for a shaddah?

May i know the rulings for a shaddah? Is it 2 consecutive same letters and the first has a sukun and the other has another vowel (fathah, kasrah or dammah)?

Let's say Al-Fatihah verse 5: is it read "iiyaa" (lengthening of the hamzah to 2 harakat) or "iyyaa"? (the merging of of hamzah and ya and no lengthening of hamzah).

Wa alaikum assalaam wa rahmatullahi wa barakaatuh
A shaddah indicates two letters merged into one, the first one is saakinah and the second has a vowel.  The shaddah is always indicating one particular letter is pronounced twice, once with a sukoon and the second with a vowel.  There are cases of one letter merging into a second letter, but we only pronounce the second letter with a shaddah, as "idhgaam" or merging means two letters have become one. 
In the example: there is a نون ساكنة merging into a راء.   We pronounce it as two راء., the first saakinah and the second with a fathah.  We do not pronounce the نون because it merged into the راء. 
In the case of إيّاك there are two ya' pronounced and the hamzah does not merge into the ya'.   The hamzah is pronounced normally with a kasrah.  The hamzah is not ever lengthened as it is not a medd letter.  Since there is a shaddah on the ya', it also is not lengthened. 
See also:
Feel free to write again if this is not clear to you.