function multicall(State storage state, bytes[] calldata data) internal returns (bytes[] memory results) {
    state.data.isMulticall = true;

    ...

    state.data.isMulticall = false;
}
function multicall(State storage state, bytes[] calldata data) internal returns (bytes[] memory results) {
+    if (state.data.isMulticall == true) revert();
    state.data.isMulticall = true;

    ...

    state.data.isMulticall = false;
}
