This is an experimental package that provides fixed footnotes; only one instance of a fixed footnote will appear on any one page. (The package is "experimental" because it's lacking at least one obvious facility; no reports of problems in actual use have been received.)
The package defines a single command, \DeclareFixedFootnote, which takes an optional *, and two `ordinary' arguments.
If the optional * is present, the command declared has an \xspace command (from the xspace package) imbedded in it, so that spaces following the fixed footnote in running text tend not to be lost.
The first ordinary argument is a command name. The second ordinary argument is the fixed text of the footnote.
\DeclareFixedFootnote will then define the command to produce the footnote, or to a link to an existing copy of the fixed footnote on the same page.
e.g., \DeclareFixedFootnote*{\sic}{\emph{sic}} \DeclareFixedFootnote{\prooflater}{This theorem will be proved later}
The package works by exchanging information with itself via the .aux file. Just as with labels (and per-page footnote numbers in footmisc), the user ordinarily needs to run latex at least twice.
The package does not (yet) have the ability to warn the user that another run of latex is still necessary. Some evidence of user demand would be necessary to encourage me to invest effort in getting LaTeX to give that warning.
Robin Fairbairns afterthought, of 2009-07-20