The footnote text is repeated several times at the bottom of the page

Question

Why is the footnote text repeated several times at the bottom of the page?

How to get the footnote text only once? (and not 6 times as in the following example)

\documentclass{article}
\usepackage[a4paper,margin=1mm]{geometry}
\usepackage{lipsum}
\usepackage{tabularray}


% https://tex.stackexchange.com/questions/645421/tabularray-with-footnotes/712683#712683
\UseTblrLibrary{counter}
\UseTblrLibrary{functional}

\makeatletter
\IgnoreSpacesOn
\tlNew \gFootNoteTl
\intNew \gFootNoteInt
\prgNewFunction \footNote {m}
  {
    \tlPutRight \gFootNoteTl
      {
        \stepcounter{footnote}
        \footnotetext{#1}
      }
    \prgReturn {\footnotemark{}}
  }
\AddToHook{env/longtblr/before}{
  \intSetEq \gFootNoteInt \c@footnote
  \tlClear \gFootNoteTl
}
\AddToHook{env/longtblr/after}{
  \intSetEq \c@footnote \gFootNoteInt
  \tlUse \gFootNoteTl
}
\IgnoreSpacesOff
\makeatother


\begin{document}

\begin{longtblr}[
  presep=-4pt,postsep=0pt
]{
  colspec = {lX},
  colsep=4pt,
  hline{odd}={solid},
  row{odd}={belowsep=0pt},
  row{even}={abovesep=0pt},
  cell{even}{1} = {c=2}{wd=\textwidth-8pt,halign=j}
}
    01/01/2020 $-$ 31/12/2022 & \textbf{First}\\
    \lipsum[2] &\\
    01/01/2018 $-$ 31/12/2019 \footNote{my footnote} & \textbf{Second}\\
    \lipsum[2] &\\
    01/01/2016 $-$ 31/12/2017 & \textbf{Third}\\
    \lipsum[2] &\\
\end{longtblr}

\end{document}

EDITED after the comments of @daleif and @gusbrs, who I thank a lot!

The moving of \prgReturn {\footnotemark{}} outside the \IfBooleanF condition, impedes the footnote symbol to go down to the following line in the table:

\documentclass{article}
\usepackage[a4paper,margin=1mm]{geometry}
\usepackage{lipsum}
\usepackage{tabularray}


% https://tex.stackexchange.com/questions/645421/tabularray-with-footnotes/712683#712683
\UseTblrLibrary{counter}
\UseTblrLibrary{functional}

\makeatletter
\IgnoreSpacesOn
\tlNew \gFootNoteTl
\intNew \gFootNoteInt
\prgNewFunction \footNote {m}
  {
    \IfBooleanF { \lTblrMeasuringBool }
      {
        \tlPutRight \gFootNoteTl
          {
            \stepcounter{footnote}
            \footnotetext{#1}
          }
        %\prgReturn {\footnotemark{}}  % <-- Before
      }
      \prgReturn {\footnotemark{}} % <-- Now
  }
\AddToHook{env/longtblr/before}{
  \intSetEq \gFootNoteInt \c@footnote
  \tlClear \gFootNoteTl
}
\AddToHook{env/longtblr/after}{
  \intSetEq \c@footnote \gFootNoteInt
  \tlUse \gFootNoteTl
}
\IgnoreSpacesOff
\makeatother


\begin{document}

\begin{longtblr}[
  presep=-4pt,postsep=0pt
]{
  colspec = {lX},
  colsep=4pt,
  hline{odd}={solid},
  row{odd}={belowsep=0pt},
  row{even}={abovesep=0pt},
  cell{even}{1} = {c=2}{wd=\textwidth-8pt,halign=j}
}
    01/01/2020 $-$ 31/12/2022 & \textbf{First}\\
    \lipsum[2] &\\
    01/01/2018 $-$ 31/12/2019 \footNote{my footnote} & \textbf{Second}\\
    \lipsum[2] &\\
    01/01/2016 $-$ 31/12/2017 & \textbf{Third}\\
    \lipsum[2] &\\
\end{longtblr}

\end{document}

Answer 1

tabularray tables perform a number of measuring passes before getting its final typeset form, so your footnote gets typeset every pass. You can test for measuring passes with \lTblrMeasuringBool:

\documentclass{article}
\usepackage[a4paper,margin=1mm]{geometry}
\usepackage{lipsum}
\usepackage{tabularray}


% https://tex.stackexchange.com/questions/645421/tabularray-with-footnotes/712683#712683
\UseTblrLibrary{counter}
\UseTblrLibrary{functional}

\makeatletter
\IgnoreSpacesOn
\tlNew \gFootNoteTl
\intNew \gFootNoteInt
\prgNewFunction \footNote {m}
  {
    \IfBooleanF { \lTblrMeasuringBool }
      {
        \tlPutRight \gFootNoteTl
          {
            \stepcounter{footnote}
            \footnotetext{#1}
          }
        \prgReturn {\footnotemark{}}
      }
  }
\AddToHook{env/longtblr/before}{
  \intSetEq \gFootNoteInt \c@footnote
  \tlClear \gFootNoteTl
}
\AddToHook{env/longtblr/after}{
  \intSetEq \c@footnote \gFootNoteInt
  \tlUse \gFootNoteTl
}
\IgnoreSpacesOff
\makeatother


\begin{document}

\begin{longtblr}[
  presep=-4pt,postsep=0pt
]{
  colspec = {lX},
  colsep=4pt,
  hline{odd}={solid},
  row{odd}={belowsep=0pt},
  row{even}={abovesep=0pt},
  cell{even}{1} = {c=2}{wd=\textwidth-8pt,halign=j}
}
    01/01/2020 $-$ 31/12/2022 & \textbf{First}\\
    \lipsum[2] &\\
    01/01/2018 $-$ 31/12/2019 \footNote{my footnote} & \textbf{Second}\\
    \lipsum[2] &\\
    01/01/2016 $-$ 31/12/2017 & \textbf{Third}\\
    \lipsum[2] &\\
\end{longtblr}

\end{document}

Note, however, that the MWE above is "proof of principle", it is not quite good enough, because I disabled the footnote completely in the measuring passes, and you actually should include the mark in those passes not to throw the measurement off by the width of the mark itself. But I did not quite understand what you were trying to do there, so I just disabled it.