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}
\endhead
contains a spurious\\
and a spurious\hline
if you remove them the corresponding output will go.! LaTeX Error: Something's wrong--perhaps a missing \item.