How can Delete the repated Hline in longtable?

Question

When I write a longtable I get a double horizontal line as shown in pictute:

the code is :

\documentclass{article}
\usepackage{longtable}

\begin{document}
\begin{longtable}[htb]{p{0.22\linewidth}p{0.5\textwidth}p{.2\textwidth}}
    \caption{my table}\\ \hline
    \multicolumn{1}{c}{\textbf{Starting material}} &
    \multicolumn{1}{c}{\textbf{Finding}} & \multicolumn{1}{c}{\textbf{Author}}\\ \hline
    \endfirsthead 
    %\vspace{-0.5\baselineskip}
    \caption[]{Continue} 
    \endhead \\ \hline

    Silver nitrate (AgNO3) polyvinylpyrrolidone & \vspace{-0.6\baselineskip}
    \begin{itemize}[noitemsep,labelsep=1ex,leftmargin=*,topsep=0pt,partopsep=0pt]%itemsep=-1ex, label
        \item The structural analysis showed that synthesized material is face-centered cubic (fcc) and monodispersed within the PVP matrix with an average size of 22 nm.
        \item The UV–vis spectrum of AgNPs in visible light region revealed that the peak at 406 nm is very broad, and significant absorption exists at >500 nm, all consistent with an aggregated silver solution. 
        \item The AgNPs film has good sensitivity in the range of 50–1200 ppm at room temperature accompanying increase of gas concentration and temperature.
    \end{itemize} \vspace{-\baselineskip}&  Kumar \\ \hline
e &r&s\\


\end{longtable}
\end{document}

Answer 1

Your code fragment has many issues. As @David Carlisle noted, your problem is caused by wrong position of the line terminator after \endfirsthead (see MWE below). Other issues are mainly off-topic since you not provide MWE (Minimal Working Example) a small but complete document document, which we can test. Consequently I added preamble to your code fragment according to my way of writing tables:

\documentclass{article}
\usepackage{array, booktabs,    % new
            longtable,
            makecell}           % new
\renewcommand\theadfont{\normalsize\bfseries}   % new
\renewcommand\theadgape{}                       % new
\usepackage[skip=1ex]{caption}                  % new
\usepackage[version=4]{mhchem}                  % new
\usepackage{siunitx}                            % new
\sisetup{range-phrase = -- ,
         range-units = single}
\usepackage{enumitem}                           % new
\usepackage{etoolbox}                           % new
\AtBeginEnvironment{longtable}{% new
\setlist[itemize]{nosep,
                 leftmargin=*,
                 label=\textbullet,
                 before=\begin{minipage}[t]{\linewidth}, 
                 after=\end{minipage}}
\setlength\tabcolsep{3pt}                   

\usepackage{lipsum}% for generating dummy text in table

\begin{document}
\begin{longtable}{@{}
        >{\raggedright\arraybackslash}p{\dimexpr 0.30\linewidth-\tabcolsep}     % changed
        >{\raggedright\arraybackslash}p{\dimexpr 0.50\textwidth-2\tabcolsep}    % changed
        >{\raggedright\arraybackslash}p{\dimexpr 0.20\textwidth-\tabcolsep}     % changed
                 @{}}
    \caption{My table}              
    \label{tab:longtable}               \\ 
    \toprule                                            % changed
    \thead{Starting\\ material}                         % changed
        &   \thead{Finding}
            &   \thead{Author}          \\ 
    \midrule
\endfirsthead
    \caption[]{My table -- Continue}    \\  % error was here
    \toprule
    \thead{Starting\\ material}
        &   \thead{Finding}
            &   \thead{Author}          \\
    \midrule
    \addlinespace[-3pt]
\endhead 
    \multicolumn{3}{r}{\footnotesize\textit{Continue on the next page}} % new
\endfoot                                    % new
    \bottomrule                             % new
\endlastfoot                                % new
% table body
    Silver nitrate (\ce{AgNO3}) polyvinylpyrrolidone 
        &   \begin{itemize}
        \item   The structural analysis showed that synthesized material is face-centered cubic (fcc) and monodispersed within the PVP matrix with an average size of 22 nm.
        \item   The UV–vis spectrum of \ce{AgNPs} in visible light region revealed that the peak at \SI{406}{nm} is very broad, and significant absorption exists at \SI{>500}{nm}, all consistent with an aggregated silver solution.
        \item   The \ce{AgNPs} film has good sensitivity in the range of \SIrange{50}{1200}{ppm} at room temperature accompanying increase of gas concentration and temperature.
            \end{itemize}
            &   Kumar               \\
e   &   r   &   s                   \\
    \addlinespace
e   & \lipsum[11]   &   s                   \\
\end{longtable}
\end{document}

(red lines indicate page layout)