lunnularia: is the n doubled?
heard as lunnularia
spelled lunularia
No. Lunularia has a single n.
What the ear did
The moon is luna, with one n, and the diminutive adds -ula without touching it. Doubling the n would put a short vowel in front of it, which is the opposite of the long u this name starts with.
L-U-N-U-L-A-R-I-A
The full entry for lunularia covers the name itself.