Polygon label repeated for each tile

Below is an example of an SLD rule that places a label at the center of a feature's geometry. This uses the ogc:Function called "centroid" to place the label. You can read more about SLD functions in the GeoServer docs, and some examples are given here.

<sld:Rule>
      <MaxScaleDenominator>5000</MaxScaleDenominator>
      <sld:TextSymbolizer>
        <sld:Geometry>
          <ogc:Function name="centroid">
            <ogc:PropertyName>the_geom</ogc:PropertyName>
          </ogc:Function>
        </sld:Geometry>
        <sld:Label>
          <ogc:PropertyName>LOT_NAME</ogc:PropertyName>
        </sld:Label>
        <sld:Font>
          <sld:CssParameter name="font-family">Arial</sld:CssParameter>
          <sld:CssParameter name="font-size">11</sld:CssParameter>
          <sld:CssParameter name="font-style">normal</sld:CssParameter>
          <sld:CssParameter name="font-weight">bold</sld:CssParameter>
        </sld:Font>
        <sld:LabelPlacement>
          <sld:PointPlacement>
            <sld:AnchorPoint>
              <sld:AnchorPointX>
                <ogc:Literal>0.0</ogc:Literal>
              </sld:AnchorPointX>
              <sld:AnchorPointY>
                <ogc:Literal>0.5</ogc:Literal>
              </sld:AnchorPointY>
            </sld:AnchorPoint>
            <sld:Rotation>
              <ogc:Literal>0</ogc:Literal>
            </sld:Rotation>
          </sld:PointPlacement>
        </sld:LabelPlacement>
        <sld:Halo>
          <sld:Radius>
            <ogc:Literal>1.0</ogc:Literal>
          </sld:Radius>
          <sld:Fill>
            <sld:CssParameter name="fill">#FFFFFF</sld:CssParameter>
          </sld:Fill>
        </sld:Halo>
        <sld:VendorOption name="conflictResolution">true</sld:VendorOption>
        <sld:VendorOption name="goodnessOfFit">0</sld:VendorOption>
        <sld:VendorOption name="autoWrap">60</sld:VendorOption>
      </sld:TextSymbolizer>
    </sld:Rule>

Also, the SLD Cookbook is a great reference. One thing that can trip you up is the ordering of tags in the SLD. For the TextSymbolizer rule above you can see required order by looking in the schema definition. Don't worry, it's not too scary! Just search for "textsymbolizer" in that .xsd file, an you should easily find the "sequence" tag. There you'll find that the element references match up with the order in my example. (Note: I didn't use the text symbolizer's "fill" attribute, my fill just applies to the halo.)