I'd be very surprised if she's double merle based on the amount of color she has on her body and the amount of pigment she has on her nose, lips, and around her eyes. Double merles are more likely to have a lot of pink/unpigmented skin in their white areas, and also tend to have significantly more white than merle overall. Genetics is tricky and I'm not familiar enough with all forms of the merle gene to say it's impossible, but my (totally amateur) opinion is that she's probably not a double.
It might be worth asking your vet if she should be checked for iris deformities. Iris colobomas, for example, are common in herding breeds, independent of double merle issues, and sound similar to what you're describing. If that's what's causing it, the issue isn't serious or degenerative, but an official diagnosis from your vet or a veterinary ophthalmologist will be the most reliable way to confirm what's going on!