A solution that does not change the order (untested):
printf("%lf\t%lf\t%lf\n",plane[0][0],plane[0][1],plane[0][2]);
for(p=1;p<count;p++)
{
for(ppp=0;ppp<p;ppp++)
{
if(plane[p][1]==plane[ppp][1])
break;
}
if (p >= ppp)
printf("%lf\t%lf\t%lf\n",plane[p][0],plane[p][1],plane[p][2]);
}