Created using remix-ide: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://remix.ethereum.org/#version=soljson-v0.5.1+commit.c8a2cb62.js&optimize=true&gist=

DocumentSigner.sol

pragma solidity >=0.4.21 < 0.6.0;

contract DocumentSigner {
    mapping(bytes32=>string) public docs;
    mapping(bytes32=>address[]) public signers;
    
    modifier validDoc(bytes32 _docHash) {
        require(bytes(docs[_docHash]).length != 0, "Document is not submitted");
        _;
    }

    event Sign(bytes32 indexed _doc, address indexed _signer);
    event NewDocument(bytes32 _docHash);

    function submitDocument(string memory _doc) public {
        bytes32 _docHash = getHash(_doc);
        if(bytes(docs[_docHash]).length == 0) {
            docs[_docHash] = _doc;
            emit NewDocument(_docHash);
        }
    }

    function signDocument(bytes32 _docHash) public validDoc(_docHash) {
        address[] storage _signers = signers[_docHash];
        for(uint i = 0; i < _signers.length; i++) {
            if(_signers[i] == msg.sender) return;
        }
        _signers.push(msg.sender);
    }
    
    function getDetail(bytes32 _docHash) public validDoc(_docHash) view returns(string memory _doc, address[] memory _signers) {
        _doc = docs[_docHash];
        _signers = signers[_docHash];
    }
    
    function getHash(string memory _doc) public pure returns(bytes32) {
        return keccak256(abi.encodePacked(_doc));
    }
}