How do I refer to the same footnote more than once?

The quick and dirty way

In the case that you just need to refer to the last added footnote, you can also simply put

\footnotemark[\value{footnote}]

in your text. But please consider the longer version as well as this one.

The long term solution

Put the following new commands in the preamble of your document

\newcommand{\footnoteremember}[2]{
  \footnote{#2}
  \newcounter{#1}
  \setcounter{#1}{\value{footnote}}
}
\newcommand{\footnoterecall}[1]{
  \footnotemark[\value{#1}]
}

You can now add footnotes in your document, and remember a reference to them with, e.g.,

\footnoteremember{myfootnote}{This is my footnote}

and then refer to this footnote again with

\footnoterecall{myfootnote}.

Note that each reference can only be used once, La Te X will generate errors when references are duplicate.