When reading UsdPreviewSurface materials from USD files, duplicate links
between nodes would often result. This typically occurs between the
Image node and its upstream UV Mapping node, or between the Image node
and a downstream Separate RGB node. As processing progresses, we
de-duplicate the nodes themselves as each new input/output socket is
discovered. However, we would unconditionally add a link between the
nodes even if we've already added one.
Cycles will complain about this situation and it's obviously incorrect:
`Cycles shader graph connect: input already connected.`
As most UsdPreviewSurface material networks are all fairly small
(<10 links total) I'm not worried about the cost of counting the links
at this point.
Pull Request: https://projects.blender.org/blender/blender/pulls/130356
e68897e117