How to duplicate in 2 x A5 a tcolorbox written in A5 format on several page?

This exploits that atbegshi, which is loaded by eso-pic, puts the contents of the page in a box, \AtBeginShipoutBox. This box can be copied. The following is a proof of principle and works regardless of tcolorbox. It does, however, assume that the user is careful enough to confine the stuff on the left side of the page. There are many ways in which one can extend this (and I am sure that someone will do that because there is no way this answer will get accepted), and I openly admit that, because of the reason, I engineered some small shifts by hand rather than finding out precisely how one may compute them from the page geometry. Also, one does not need necessarily to use TikZ here, the mechanisms of eso-pic suffice, but you are loading tcolorbox so this is not much extra. At any rate, this example shows the principle: add a hook that copies the ship out box to the right half of the document.

\documentclass[a4paper, 11pt,landscape]{article}
\usepackage[breakable]{tcolorbox}
\usepackage{lipsum}
\usepackage{eso-pic}
\usepackage{tikz}
\makeatletter
\AddToShipoutPictureFG{\begin{tikzpicture}[overlay,remember picture]
\node at ([xshift=\textwidth/4+2em,yshift=0.9em]current page.center) {\copy\AtBeginShipoutBox};
\end{tikzpicture}}
\makeatother
\pagestyle{empty}
\begin{document}
\foreach \X [count=\Y] in {Duck,Marmot,Koala,Bear,Cat}
{\section{\X}
\begin{tcolorbox}[breakable,width=0.46\textwidth]
  \lipsum[\Y]
\end{tcolorbox}}
\end{document}

(I should mention that there is software that does the same to a pdf file. At least that's what I believe, in the old days that was some postscript command, maybe psnup, maybe not, which allowed one to do that for ps files.)

ADDENDUM: There seems to be a misunderstanding: this solution does not produce as many boxes as pages. To make this even clearer, I add another example.

\documentclass[a4paper, 11pt,landscape]{article}
\usepackage[breakable,skins]{tcolorbox}
\usepackage{lipsum}
\usepackage{eso-pic}
\usepackage{tikz}
\makeatletter
\AddToShipoutPictureFG{\begin{tikzpicture}[overlay,remember picture]
\node at ([xshift=\textwidth/4+2em,yshift=0.9em]current page.center) {\copy\AtBeginShipoutBox};
\end{tikzpicture}}
\makeatother
\pagestyle{empty}
\begin{document}
\section{One very long box}
\begin{tcolorbox}[enhanced jigsaw,breakable,width=0.46\textwidth]
  \lipsum[1-5]
\end{tcolorbox}
\end{document}

What could this be used for? Just duplication of the left column seems a bit boring to me. The point here is that you can play with TikZ. Here is an application that mirrors the left column, but obviously you could rotate it, change the opacity or even overt it (using fadings).

\documentclass[a4paper, 11pt,landscape]{article}
\usepackage[breakable,skins]{tcolorbox}
\usepackage{lipsum}
\usepackage{eso-pic}
\usepackage{tikz}
\makeatletter
\AddToShipoutPictureFG{\begin{tikzpicture}[overlay,remember picture]
\node at ([xshift=\textwidth/4+2em,yshift=0.9em]current page.center) 
[xscale=-1]{\copy\AtBeginShipoutBox};
\end{tikzpicture}}
\makeatother
\pagestyle{empty}
\begin{document}
\section{One very long box}
\begin{tcolorbox}[enhanced jigsaw,breakable,width=0.46\textwidth]
  \lipsum[1-5]
\end{tcolorbox}
\end{document}