Welcome to the forums.
It is a spelling matter - see here, and here
Anglika I know all these rules (Words with Double Consonants). This is General Grammar.
I need a (morphological) notion for this case.
When dividing a word, we have prefix - root - suffix - ending (that is a kind of suffix)
I wanted to know this second Consonant refers to the root or is it a suffix ?
running
runn = root
ing = ending
run = root
n = suffix
ing = ending
Suffix is the part of the word that comes after the root.
So, the second
- n - comes after the root
run, or it is a part of it?
Or may be there is a special term for it.