使用ERC20铸造ERC721 NFT时遇标识符重复声明错误求助

我是Solidity新手，正在尝试实现支持ETH及USDT等ERC20代币购买铸造ERC721 NFT的功能，但无论如何尝试，始终出现如下错误：

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

我的代码如下：

// 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);
    }
}

问题原因及解决方案 #

1. 核心错误：导入版本不一致 #

你导入OpenZeppelin合约时混用了带版本号（@4.8.0）和不带版本号的路径：

• 前5个导入用的是@openzeppelin/contracts@4.8.0/...
• 最后一个ERC20导入用的是@openzeppelin/contracts/token/ERC20/ERC20.sol

这会导致编译器加载两个不同版本的ERC20合约，从而触发Identifier already declared错误。

2. 修复导入问题 #

将所有OpenZeppelin合约的导入路径统一版本号，修改最后一行导入为：

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

3. 额外代码问题修正 #

除了导入错误，你的代码还有两处需要调整：

• buyMembershipUsdt函数中使用了未定义的_amount变量，应替换为usdtPrice
• 该函数不需要payable修饰符，因为USDT转账和ETH转账是独立逻辑，添加payable会允许用户误转ETH到合约

修正后的函数：

function buyMembershipUsdt() external {
    tokenUsdt.safeTransferFrom(msg.sender, owner(), usdtPrice);
    mintMembership();
}

4. 优化建议：使用IERC20接口 #

由于你只是调用USDT的标准ERC20方法，不需要继承或实现完整的ERC20合约，推荐导入IERC20接口来减少合约字节码大小：

import "@openzeppelin/contracts@4.8.0/token/ERC20/IERC20.sol";

同时将变量声明改为：

IERC20 public tokenUsdt;

内容的提问来源于stack exchange，提问作者MuthaFury