The duff-porter composite through the SLD VendorOption doesn't work well when a tiled WMS getMap call is requested. Single-tile works as expected.
We have two FeatureStyles that we composite with a destination-in operation to color linestrings only when they are intersecting a polygon, but if we zoom in sufficiently where no linestrings are present, parts of the polygons appear (cut in a way that follows tiling).
This seems to lead to a bug in the way the tiling works.
The two screenshots show the same area (at different zoom levels), and you see the polygons appear on each side of the linestrings.
OS X Yosemite, Ubuntu 14.04