contract A {
	int public testvar;
	function foo() internal returns (uint256);
	function test(uint8 _a) internal returns (uint256);
}
contract B {
	function foo() internal returns (uint256);
	function test() internal returns (uint256);
}
contract C {
	function foo() internal returns (uint256);
}
contract D {
	function foo() internal returns (uint256);
	function test() internal returns (uint256);
}
contract X is A, B, C, D {
	int public override testvar;
	function test() internal override(B, D, D) returns (uint256);
	function foo() internal override(A, C, B, B, B, D ,D) returns (uint256);
}
// ----
// TypeError: (498-499): Duplicate contract "D" found in override list of "test".
// TypeError: (563-564): Duplicate contract "B" found in override list of "foo".
// TypeError: (566-567): Duplicate contract "B" found in override list of "foo".
// TypeError: (572-573): Duplicate contract "D" found in override list of "foo".