I created my thesis and used this code to created heading. The document is setted to book class and oneside option. I got a page where both the left and right are number. Instead I would have the page number at the right and at the left the section

\usepackage{fancyhdr}
\pagestyle{fancy}

\makeatletter
\DeclareRobustCommand{\format@sec@number}[2]{{\normalfont\upshape#1}#2}
\renewcommand{\chaptermark}[1]{%
\markboth{\format@sec@number{\ifnum\c@secnumdepth>\m@ne\@chapapp\ \thechapter. \fi}{#1}}{}}
\renewcommand{\sectionmark}[1]{%
\markright{\format@sec@number{\ifnum\c@secnumdepth>\z@\thesection. \fi}{#1}}}
\makeatother

\fancyhf{}

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You are defining both side heading to be page numbers via

\fancyhead[L,R]{\thepage}


hence you get the page numbers on both sides. Try the following code:

\documentclass[oneside]{book}
\usepackage{lipsum}
\usepackage{fancyhdr}
\pagestyle{fancy}

\makeatletter
\DeclareRobustCommand{\format@sec@number}[2]{{\normalfont\upshape#1}#2}
\renewcommand{\chaptermark}[1]{%
\markboth{\format@sec@number{\ifnum\c@secnumdepth>\m@ne\@chapapp\ \thechapter. \fi}{#1}}{}}
\renewcommand{\sectionmark}[1]{%
\markright{\format@sec@number{\ifnum\c@secnumdepth>\z@\thesection. \fi}{#1}}}
\makeatother

\fancyhf{}