My experience (I'm a non-identical twin, so I have a bit!) is that people with a degree in Spanish philology do make a distinction and most other people just call all twins gemelos. I've witnessed a native speaker call identical twins mellizos, and I've used "mellizo" with an Ecuadorian friend and then had to explain to him what it means.
Basically, it's like español vs castellano: whichever you use, you'll be wrong.
|