How common is it...

[Greensleeves]

..for parents to insist that their child's double-barrelled name be spoken in full each time the child is addressed?

It's quite new to me but I seem to be coming across it more and more

I know a Thierry-Leanne, a Freya-Jayne and a Tiana-Louise, all of whomuse the full name rather than just the first

not casting aspersions, just wondering how common it is now

I don't think the OP meant 'common as muck' common, but 'widespread' common. Didn't she?

I'd always try to call people by the name they (or their parents) prefer, but I think names of more than 3 syllables are going to get shortened sooner or later.

I think its normal, all the double first names at my school were introduced as such.
Having said that, they were fairly short names, like Anne-Marie, not long ones as you mention. I think when the names are long, it can sound a bit poncy.

Know a bunch of Sarah-Janes' and all of them are known as 'SJ'