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OurSBT.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/[email protected]/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/[email protected]/access/Ownable.sol";
import "@openzeppelin/[email protected]/utils/Counters.sol";

contract OurSBT is ERC721Enumerable, Ownable {
    using Counters for Counters.Counter;

    Counters.Counter private _tokenIdCounter;

    constructor() ERC721("OurSBT", "OST") {}

    function safeMint(address to) public payable onlyOwner {
        require(balanceOf(to) == 0, "Already Minted to Address!");

        uint256 tokenId = _tokenIdCounter.current();
        _tokenIdCounter.increment();
        _safeMint(to, tokenId);
    }

    // The following functions are overrides required by Solidity.

    function _beforeTokenTransfer(address from, address to, uint256 tokenId)
        internal
        override
    {
        super._beforeTokenTransfer(from, to, tokenId);
        require(from == address(0), "Not Allowed");
    }


    function supportsInterface(bytes4 interfaceId)
        public
        view
        override
        returns (bool)
    {
        return super.supportsInterface(interfaceId);
    }
}

Soul Bound Tokens
Above is an implementation of Early stage SBT created by modifying ERC721.