One d or two?
heard as claddonia
spelled cladonia
One. claddonia should be Cladonia.
What the ear did
English doubles a consonant to keep the vowel in front of it short, which is why we write ladder, adder and sudden. The a in Cladonia really is short, so the instinct is sound and the spelling still refuses it: the d belongs to klados and stands alone.
C-L-A-D-O-N-I-A
The full entry for cladonia covers the name itself.