gluTessXXXX problem

Hi, I draw polygons in a openGL scene with this functions.
GLUtesselator * pTess = gluNewTess();
gluTessCallback(…, glBegin);
gluTessCallback(…, glVertex3dv);
gluTessCallback(…, glEdgeFlag);
gluTessCallback(…, glEnd);

gluTessBeginPolygon(pTess, NULL);
for ()
gluTessVertex(pTess, pPts+j, pPts+j);

the fill-polygon are drawn ok. But if there are some layers with fill-polygons which overlaps (but equal z-coordinate), the new polygons are not drawn fully above the old polygons. The scene shows some areas where the lower polygons are draw above the last polygons.

How do I resolve this error ???
thank you very much