Vtgyjfy

I have 2 almost idenctical lines of LaTeX source.
They differ only in switching order of these two lines

usepackage[fleqn]{amsmath}



usepackage{amsmath,amssymb}

One causes error and the other one compiles. WHY?

NO ERROR:

documentclass{article}

usepackage[fleqn]{amsmath}





usepackage{amsmath,amssymb}





usepackage{imakeidx}



makeindex



begin{document}



Some text



index{Apple}index{Apricot}



printindex



end{document} 

ERROR:

documentclass{article}





usepackage{amsmath,amssymb}

usepackage[fleqn]{amsmath}





usepackage{imakeidx}



makeindex



begin{document}



Some text



index{Apple}index{Apricot}



printindex



end{document}

The second source causes this error:

Command Line:   bibtex.exe "ideje10"

Startup Folder: C:Usershynek0DesktopTH0dipl



This is BibTeX, Version 0.99d (MiKTeX 2.9 64-bit)

The top-level auxiliary file: ideje10.aux

I found no citation commands---while reading file ideje10.aux

I found no bibdata command---while reading file ideje10.aux

I found no bibstyle command---while reading file ideje10.aux

(There were 3 error messages)





Command Line:   makeindex.exe "ideje10.idx"

Startup Folder: C:Usershynek0DesktopTH0dipl



This is makeindex, version 2.15 [MiKTeX 2.9 64-bit] (kpathsea + Thai support).

Scanning style file ./ideje10.mst...done (3 attributes redefined, 0 ignored).

Scanning input file ideje10.idx....done (2 entries accepted, 0 rejected).

Sorting entries....done (2 comparisons).

Generating output file ideje10.ind....done (7 lines written, 0 warnings).

Output written in ideje10.ind.

Transcript written in ideje10.ilg.





Command Line:   pdflatex.exe --interaction=errorstopmode --synctex=-1 "C:Usershynek0DesktopTH0diplideje10.tex"

Startup Folder: C:Usershynek0DesktopTH0dipl



This is pdfTeX, Version 3.14159265-2.6-1.40.15 (MiKTeX 2.9 64-bit)

entering extended mode

(C:/Users/hynek0/Desktop/TH0dipl/ideje10.tex

LaTeX2e <2014/05/01>

Babel <3.9l> and hyphenation patterns for 68 languages loaded.

(D:ProgramFilesMikTexExetexlatexbasearticle.cls

Document Class: article 2014/09/29 v1.4h Standard LaTeX document class

(D:ProgramFilesMikTexExetexlatexbasesize10.clo))

(D:ProgramFilesMikTexExetexlatexamsmathamsmath.sty

For additional information on amsmath, use the `?' option.

(D:ProgramFilesMikTexExetexlatexamsmathamstext.sty

(D:ProgramFilesMikTexExetexlatexamsmathamsgen.sty))

(D:ProgramFilesMikTexExetexlatexamsmathamsbsy.sty)

(D:ProgramFilesMikTexExetexlatexamsmathamsopn.sty))

(D:ProgramFilesMikTexExetexlatexamsfontsamssymb.sty

(D:ProgramFilesMikTexExetexlatexamsfontsamsfonts.sty))



! LaTeX Error: Option clash for package amsmath.



See the LaTeX manual or LaTeX Companion for explanation.

Type  H <return>  for immediate help.

 ...                                              



l.6 



? 



! Emergency stop.

 ...                                              



l.6 

Process has been killed ...

===========================================
I can of course switch the lines to get a compiling code, but I'm curious why this happens.

From the documentation of usepackage in source2e.pdf:

usepackage[⟨package-option-list ⟩]{⟨package-list ⟩}[⟨version ⟩]

There
can be any number of these declarations. All packages in
⟨package-list⟩ are called with the same options. [...]

Each
package is loaded only once. If the same package is requested more
than once, nothing happens, unless the package has been requested with
options that were not given the first time it was loaded, in which
case an error is produced.

In your second example you load amsmath with the option fleqn that was not given the first time, so you get an error ("option clash"). In general each package should only be loaded once.