我在编译 .bib 文件后遇到了这个错误。请问这个错误是什么意思？如何找出错误所在？此外，我发现所有引用都多次定义。但是，在 .bib 文件中，每个引用只出现一次。
This is BibTeX, Version 0.99d (TeX Live 2017/Debian)
The top-level auxiliary file: thesis.aux
A level-1 auxiliary file: title.aux
A level-1 auxiliary file: preface.aux
The style file: plainnat.bst
A level-1 auxiliary file: chapter1.aux
A level-1 auxiliary file: chapter2.aux
A level-1 auxiliary file: chapter3.aux
A level-1 auxiliary file: chapter4.aux
A level-1 auxiliary file: chapter5.aux
A level-1 auxiliary file: chapter6.aux
A level-1 auxiliary file: chapter7.aux
A level-1 auxiliary file: epilog.aux
A level-1 auxiliary file: bibliography.aux
Illegal, another \bibstyle command---line 3 of file bibliography.aux :
\bibstyle : {plainnat} I'm skipping whatever remains of this command
Illegal, another \bibdata command---line 4 of file bibliography.aux :
\bibdata : {bibliography} I'm skipping whatever remains of this command
Database file #1: bibliography.bib
(There were 2 error messages)
Process exited with error(s)