Github FE: Import from github doesn't work second time

[LianaHus]

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol";

pragma solidity >=0.4.22 <0.6.0;

contract testOZ is ERC20 {
}

works just fine only first time. second time I'm importing a contract from github I don't have in in FE