Question: Expansion issues

when I try to expand the polynomial (xy - 2*y + 3*z)^2*(3*x - 4*xy - 6*z + 8*yz)^2 I get an expansion where variables are repeseted multiple times like

 9*xy^2*x^2 - 36*xy*y*x^2 + 54*xy*z*x^2 + 36*y^2*x^2 - 108*y*z*x^2 + 81*z^2*x^2 - 24*xy^3*x + 96*xy^2*y*x + 48*xy^2*yz*x - 180*xy^2*z*x - 96*xy*y^2*x - 192*xy*y*yz*x + 432*xy*y*z*x + 288*xy*yz*z*x - 432*xy*z^2*x + 192*y^2*yz*x - 144*y^2*z*x - 576*y*yz*z*x + 432*y*z^2*x + 432*yz*z^2*x - 324*z^3*x + 16*xy^4 - 64*xy^3*y - 64*xy^3*yz + 144*xy^3*z + 64*xy^2*y^2 + 256*xy^2*y*yz - 384*xy^2*y*z + 64*xy^2*yz^2 - 480*xy^2*yz*z + 468*xy^2*z^2 - 256*xy*y^2*yz + 192*xy*y^2*z - 256*xy*y*yz^2 + 1152*xy*y*yz*z - 720*xy*y*z^2 + 384*xy*yz^2*z - 1152*xy*yz*z^2 + 648*xy*z^3 + 256*y^2*yz^2 - 384*y^2*yz*z + 144*y^2*z^2 - 768*y*yz^2*z + 1152*y*yz*z^2 - 432*y*z^3 + 576*yz^2*z^2 - 864*yz*z^3 + 324*z^4

how do i fix this so each variable is only shown once, ie for 9x*y^2*x^2 is shown as 9x^3*y^2