c++ error C2196: case value '1' already used

#include <iostream.h>
void main()
{
char grade;
cout<<"请输入考核成绩（A~E):"<<endl;
cin>>grade;
switch (grade)
{
case 'a'||'A' :cout<<"90~100"<<endl;break;
case 'b'||'B' :cout<<"80~89"<<endl;break;
case 'c'||'C' :cout<<"70~79"<<endl;break;
case 'd'||'D' :cout<<"60~69"<<endl;break;
case 'e'||'E' :cout<<"0~59"<<endl;break;
default :cout<<"error"<<endl;
}
}