Check out GeoWebCache, the tile cache engine that comes with GeoServer. You can specify the gutter parameter. According to the docs: "the gutter parameter is specified in pixels and represents extra padding around the image that is sliced away when the tiles are created. Certain WMS server have edge effects that can be eliminated this way, but it can also result in labels being cut off". Particularly the last statement indicates this may solve your use case.
Edward Mac Gillavry
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