DD is an older Ellie - before it became V popular, but it's use seems to have died down a bit now.
I've never really understood the "proper name" argument.
"If you like Ellie, then why not go for Eleanor, Ella etc..." Err here's an off the wall suggestion. If you like Ellie, why not have Ellie? Are all Lucy's called Lucinda? No! Are all Emma's called Emmaline? No!
Don't even get me started on the, "you need a spare name for when they become a barrister"...
"Mummy, why have I got three names?"
"Ooh well we gave you a proper name for just in case you became a judge, then we gave you a name we actually liked, then we gave you the one for worst case scenario and you started selling drugs outside schools"
What I'm trying to say is like a name for what it is. If you want a "proper name" choose one you like, don't just lengthen something else for appearances.
My friend has a Sam. Lovely name. His birth certificate says Samuel. "I'll never use it" she said, "I prefer Sam, so we'll call him Sam, but Samuel looks better on the birth certificate" Wtf?