GLSL node: preventing inputs of a certain type


I'm currently implementing a "color" node (basic post process film grain) and am hitting this error when the node has an input that is another instance of the same type:

0(183) : error C1038: declaration of "permTexUnit1e9f9bd3" conflicts with previous declaration at 0(47)<!--EndFragment-->

[Image Seq] --> [my node] --> [my node]


"permTexUnit" is a constant that I've defined in the glsl file. In practice my users won't actually hit this, since I'll make sure there's only one of these nodes in the session via my Python plugin. But is there a way to specify in the .gto file that this node can't be "chained"? Just trying to figure out the correct way to do things.