Ensure that we don't get duplicated footnotes when
a note occurs in a header cell and `\endfirsthead`
is used.

```
% pandoc -t latex
| x | y[^fn] |
|-|-|
|1|2|
: a table

[^fn]: a footnote
^D
\begin{longtable}[]{@{}ll@{}}
\caption{a table}\tabularnewline
\toprule\noalign{}
x & y\footnote{a footnote} \\
\midrule\noalign{}
\endfirsthead
\toprule\noalign{}
x & y{} \\
\midrule\noalign{}
\endhead
\bottomrule\noalign{}
\endlastfoot
1 & 2 \\
\end{longtable}
```
