Certain fron/backcolor configuration create faulty shaders

For example with vertex-program-two-side enabled front front2 back2 shader compilation fails with

vrend_compile_shader: context error reported 1 "vertex-program-two-side" Illegal shader 0
shader failed to compile
0:7(11): error: `ex_c1' redeclared
0:8(11): error: `ex_bc1' redeclared

vs: 3 GLSL:
   1: #version 140
   2: in vec4 in_0;
   3:                            out vec4 ex_bc0;
   4:                            out vec4 ex_bc1;
   5:                            out vec4 ex_c1;
   6:                              out  vec4 ex_c0;
   7:                              out  vec4 ex_c1;
   8:                              out  vec4 ex_bc1;
   9: uniform float winsys_adjust_y;
  10: void main(void)
  11: {
  12: gl_Position = vec4(((in_0)));
  13: ex_bc1 = vec4(((vec4(0,0,0.25,0))));
  14: ex_bc1 = clamp(ex_bc1, 0.0, 1.0);
  15: ex_c1 = vec4(((vec4(0,0.25,0,0))));
  16: ex_c1 = clamp(ex_c1, 0.0, 1.0);
  17: ex_c0 = vec4(((vec4(0,0.5,0,0))));
  18: ex_c0 = clamp(ex_c0, 0.0, 1.0);
  19: gl_Position.y = gl_Position.y * winsys_adjust_y;
  20: }