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
x & y\footnote{a footnote}\tabularnewline
\midrule
\endfirsthead
\toprule
x & y{}\tabularnewline
\midrule
\endhead
1 & 2\tabularnewline
\bottomrule
\end{longtable}
```
