Repeating Numbered Items in Beamer

You could use the refcount package to have an expandable reference number that you can play with. This allows you to use the tricks of your first code in the last one.

\documentclass[pdf]{beamer}
\usepackage[utf8]{inputenc}
\usepackage{tipa}
\usepackage{refcount}
\usecolortheme{beaver}
\setbeamercovered{highly dynamic}
\mode<presentation>{}

\begin{document}
   \begin{frame}
      \begin{enumerate}
      \item First.\label{1}
      \item Second.\label{2}
      \item Third.\label{3}
      \end{enumerate}
   \end{frame}
  \begin{frame}
      \begin{enumerate}\setcounter{enumi}{\the\numexpr\getrefnumber{1}-1}
      \item First again.\setcounter{enumi}{\the\numexpr\getrefnumber{3}-1}
      \item Third again.
      \end{enumerate}
  \end{frame}
\end{document}

Obviously you can cast this into a macro.

\documentclass[pdf]{beamer}
\usepackage[utf8]{inputenc}
\usepackage{tipa}
\usepackage{refcount}
\usecolortheme{beaver}
\setbeamercovered{highly dynamic}
\mode<presentation>{}
\newcommand{\repeateditem}[1]{%
\setcounter{enumi}{\the\numexpr\getrefnumber{#1}-1}%
\item}
\begin{document}
   \begin{frame}
      \begin{enumerate}
      \item First.\label{1}
      \item Second.\label{2}
      \item Third.\label{3}
      \end{enumerate}
   \end{frame}
  \begin{frame}
      \begin{enumerate}
      \repeateditem{1} First again.
      \repeateditem{3} Third again.
      \end{enumerate}
  \end{frame}
\end{document}