Unable to use ERC20

Smart Contracts
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1

I'm new in Solidity and trying to allow purchasing using ETH and other token such as USDT but I keep getting this error no matter what I tried.

        DeclarationError: Identifier already declared. --> contract-9b5b02c5de.sol:9:1:
    |
    9 | import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

This is my code:

    // SPDX-License-Identifier: MIT
    pragma solidity ^0.8.9;
    
    import "@openzeppelin/contracts@4.8.0/token/ERC721/ERC721.sol";
    import "@openzeppelin/contracts@4.8.0/token/ERC721/extensions/ERC721Enumerable.sol";
    import "@openzeppelin/contracts@4.8.0/token/ERC721/extensions/ERC721URIStorage.sol";
    import "@openzeppelin/contracts@4.8.0/access/Ownable.sol";
    import "@openzeppelin/contracts@4.8.0/utils/Counters.sol";
    import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
    
    
    contract Combat is ERC721, ERC721Enumerable, ERC721URIStorage, Ownable {
        using Counters for Counters.Counter;
    
        Counters.Counter private _tokenIdCounter;
    
        ERC20 public tokenUsdt;
        uint256 public ethPrice = 0;
        uint256 public usdtPrice = 0;
        uint256 public maxSupply; //Maximum amount token
        bool public isMintEnabled; //default : false
        mapping(address=>uint256) public mintedWallets;
    
    
        constructor(address _tokenUsdt) ERC721("XXX", "QAZ") {
            maxSupply = 100;
            tokenUsdt = ERC20(_tokenUsdt);
        }
    
        function setToken(address _tokenUsdt) external onlyOwner {
            tokenUsdt = _tokenUsdt;
        }
    
        function setPrice(uint256 _ethPrice, uint256 _usdtPrice) external onlyOwner {
            ethPrice = _ethPrice;
            usdtPrice = _usdtPrice;
        }
    
        function buyMembershipUsdt() external payable {
            require(msg.value >= usdtPrice, "Price Error");
            tokenUsdt.safeTransferFrom(msg.sender, owner(), _amount);
            mintMembership();
        }
    
        function buyMembershipEth() external payable {
            require(msg.value >= ethPrice, "Price Error");
            mintMembership();
        }
    
        function mintMembership() internal {
            require(isMintEnabled, "Not For Sale");
            require(totalSupply() < maxSupply, "Sold Out");
    
            uint256 tokenId = _tokenIdCounter.current();
            _tokenIdCounter.increment();
            _safeMint(msg.sender, tokenId);
        }
    
        function setMintEnabled(bool isMintEnabled_) external onlyOwner {
            isMintEnabled = isMintEnabled_;
        }
    
        function _baseURI() internal pure override returns (string memory) {
            return "ipfs://bafybeiebgbvibloa3p3vge7ecxobwxxnuyg4pdozbcfjigfglhz2ogidq4/";
        }
    
        function setMaxSupply(uint256 maxSupply_) external onlyOwner{
            maxSupply = maxSupply_;
        }
    
        function withdraw() public onlyOwner {
            require(address(this).balance > 0, "Balance is 0");
            payable(owner()).transfer(address(this).balance);
        }
    
        function withdrawToken() external onlyOwner {
            uint256 tokenBalance = tokenUsdt.balanceOf(address(this));
            require(tokenBalance > 0, "Insufficient balance");
            tokenUsdt.safeTransfer(msg.sender, tokenBalance);
      }
    
      // The following functions are overrides required by Solidity.
    
        function tokenURI(uint256 tokenId)
            public
            view
            override(ERC721, ERC721URIStorage)
            returns (string memory)
        {
            return super.tokenURI(tokenId);
        }
    }

I seen some guides our there but none seems to help because it uses custom token which I'm not using it. Regardless whether I'm using IERC20 or ERC20, I will keep getting error above I mentioned. Am I missing something or what?

2

Hi, welcome to the community! :wave:

I think you should use IERC20 rather than ERC20, and import

import "@openzeppelin/contracts@4.8.0/token/ERC20/utils/SafeERC20.sol";

and add using SafeERC20 for IERC20;

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