Beamer: itemize in presentation mode with lemma duplicates slide

Question

Consider this MWE:

\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{itemize}[<+->]
    \item Next up a lemma.
    \item \begin{lemma}A lemma.\end{lemma}
\end{itemize}
\end{frame}
\end{document}

I would expect to get a pdf file with two pages, but the first page is duplicated and in fact I receive three pages.

What can I do? I noticed that

\item Something: \begin{lemma}A lemma.\end{lemma}

is split up across two slides, so <+-> might interfere with lemma.

Answer 1

One possibility is to omit the second \item. In your example, this seems the right option to me:

\documentclass{beamer}
\begin{document}
  \begin{frame}
    \begin{itemize}[<+->]
      \item Next up a lemma.
      \begin{lemma}A lemma.\end{lemma}
    \end{itemize}
  \end{frame}
\end{document}

Alternatively, if you need the \item (e.g. for the \item Something case), you can explicitly override the overlay increment for the lemma:

\documentclass{beamer}
\begin{document}
  \begin{frame}
    \begin{itemize}[<+->]
      \item Next up a lemma.
      \item Something
      \begin{lemma}<.->A lemma.\end{lemma}
    \end{itemize}
  \end{frame}
\end{document}