Some more tests.

test/libsolidity/syntaxTests/fallback/inheritance_multi_base.sol

@@ -0,0 +1,20 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+}
+contract B {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return "xyz";
+    }
+}
+contract C is B, A {
+     function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// TypeError 6480: (229-420): Derived contract must override function "". Two or more base classes define function with same name and parameter types.