I was recently asking my OB about single vs. double layer sutures. (I did have a double layer with my csec).
She said: The studies that are out there do not show a difference in rupture rates between double vs. single layer. The second layer, if you have a double layer, is an embrocation (sp?) stitch, meaning that it isn't really holding the uterus closed. Instead, it's really just grabbing the very top layer of the uterus, and not structurally important. They usually only do them, if the incision is still oozing blood after they stitched it up. It's really meant to stop bleeding, not to prevent future ruptures.
Now, I haven't really looked into the differences between single vs. double layer myself, since I did have a double, but I really don't think there's much difference. I think that the biggest problem is an OB using it as an excuse to not participate in a VBAC.