I use titlesec package to set part style, but I also would like to put the name of the part in header. I try the way proposed in this thread (How to get part name in LaTeX ?). I get the header as I want but it causes a duplication of part name in my document (see below)...

Here is a part of the code :
\titleformat{\part}[display]%
{\Huge\sffamily\bfseries}%
{\filleft{\huge\sffamily \textbf{\partname \\}}}%
{0mm}%
{\thispagestyle{empty}\filleft}%
[]%
\titlespacing{\part}{0mm}{40pt}{40pt}[- \marginparwidth - \marginparsep]%
\let\letpart\part
\renewcommand{\partmark}{}
\renewcommand{\part}[2][]{%
\ifx\\#1\\
\letpart{#2}%
\renewcommand\partmark{\partname{} -- #2}%
\else
\letpart[#1]{#2}%
\renewcommand\partmark{\partname{} -- #1}%
\fi
}
I have the same problem if I just use :
\let\letpart\part
\renewcommand{\part}[1]{\letpart{#1}}
(so it seems the problem comes from the use of \let). Has anybody an idea to avoid it ? Thanks for your help.
Edit : Here is a compilable code. It is a part of a class I made ; I let the whole list of package in case you see a conflict.
\documentclass[twoside, openright]{report}
\usepackage[paper=a4paper, lmargin=15mm, rmargin=15mm, tmargin=15mm, bmargin=15mm,
includeall, marginparwidth=44mm, headheight=\baselineskip]{geometry}
\usepackage[francais]{babel}
\usepackage[latin1]{inputenc}
\usepackage[fleqn]{amsmath}
\usepackage{amssymb, mathrsfs, stmaryrd, gensymb}
\usepackage[upright]{fourier}
\usepackage{emerald}
\usepackage{setspace}
\usepackage{calc}
\usepackage{array, multirow, tabularx}
\usepackage{xcolor, color, colortbl}
\usepackage{graphicx, picins}
\usepackage{subfig}
\usepackage[top]{mcaption}
\usepackage{fancyhdr, lastpage}
\usepackage[fit]{truncate}
\usepackage{mdwlist}
\usepackage{titlesec, titletoc}
\usepackage{ulem}
\usepackage{float}
\usepackage[globalcitecopy]{bibunits}
\usepackage{multido}
\usepackage{ragged2e}
\usepackage{marginfix}
\usepackage[geometry]{ifsym}
\usepackage{ifthen}
\usepackage[strict]{changepage}
\usepackage{framed}
\makeatletter
\definecolor{abricot1}{rgb}{1,0.580,0.412}
\definecolor{abricot2}{rgb}{1,0.680,0.565}
\definecolor{abricot3}{rgb}{1,0.933,0.890}
\newcommand{\couleurP}[0]{abricot1}%
\newcommand{\couleurS}[0]{abricot2}%
\newcommand{\couleurT}[0]{abricot3}%
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0pt}
\fancyheadoffset[RO,LE]{\marginparsep + \marginparwidth}
\fancyhf{}
\fancyhead[LE]{\small\textsf{\thepage} \quad%
\textcolor{\couleurP}{$\blacksquare\thinspace$}\textcolor{\couleurS}{$\blacksquare\thinspace\blacksquare$}}
\fancyhead[RO]{\textcolor{\couleurS}{$\blacksquare\thinspace\blacksquare$}\textcolor{\couleurP}{$\thinspace\blacksquare$} \quad%
\small\textsf{\thepage}}
\fancyfoot{}
\titleformat{\part}[display]%
{\Huge\sffamily\bfseries}%
{\filleft{\huge\sffamily \textbf{\partname \\}}}%
{0mm}%
{\thispagestyle{empty}\filleft}%
[]%
\titlespacing{\part}{0mm}{40pt}{40pt}[- \marginparwidth - \marginparsep]%
\let\letpart\part
\renewcommand{\partmark}{}
\renewcommand{\part}[2][]{%
\ifx\\#1\\
\letpart{#2}%
\renewcommand\partmark{\partname{} -- #2}%
\else
\letpart[#1]{#2}%
\renewcommand\partmark{\partname{} -- #1}%
\fi
}
\makeatother
\title{{\'E}tude expérimentale...}
\fancyhead[RE]{\small\textsf{\truncate{120mm}{\nouppercase{\leftmark}}}}
\fancyhead[LO]{\small\textsf{\truncate{120mm}{{\'E}tude expérimentale...}}}
\pagestyle{empty}
\author{Emmanuel G.}
\date{\today}
\begin{document}
\pagestyle{fancy}
\maketitle
\part{Du microscopique au mésoscopique}
\fancyhead[LO]{\small\textsf{\truncate{120mm}{\partmark}}}
tba.
\end{document}






\partmarkcommand should set\leftmark. Can you tell how the page headings should appear? – egreg Dec 26 '14 at 11:57