一个code year可以定位到唯一的一个相应数据，如果某code year有n个相同的数据，xtset code year就会显示红字"数据有重复值"，duplicates drop就是把相同的n个数据只保留唯一一个。
我个人来说，重复值出现的原因有二：一是数据库本身有重复值；二是公司-年度层面数据是由个体-年度层面数据处理得来。

crup.com.cn/PerCenter/Certificate
the duplicates drop command to drop the duplicate observations.  The command drops all observations except the first occurrence of each group with duplicate observations.


简单例子测试

命令帮助里面也写了

[D] duplicates -- Report, tag, or drop duplicate observations
                  (View complete PDF manual entry)
Syntax

    Drop duplicates
        duplicates drop [if] [in]
        duplicates drop varlist [if] [in] , force

Description

    duplicates reports, displays, lists, tags, or drops duplicate observations,
    depending on the subcommand specified.  Duplicates are observations with
    identical values either on all variables if no varlist is specified or on a
    specified varlist.

    duplicates report produces a table showing observations that occur as one or
    more copies and indicating how many observations are "surplus" in the sense
    that they are the second (third, ...) copy of the first of each group of
    duplicates.

    duplicates tag generates a variable representing the number of duplicates for each observation.  This will be 0 for all unique observations.

    duplicates drop drops all but the first occurrence of each group of duplicated observations.  The word drop may not be abbreviated.

    Any observations that do not satisfy specified if and/or in conditions are
    ignored when you use report, examples, list, or drop.  The variable created
    by tag will have missing values for such observations.

duplicates drop drops all but the first occurrence of each group of duplicated observations.  The word drop may not be abbreviated.
谢谢各位！简单用例子 stata试验了下。 一组code 与 year duplicates只保留第一个。

那么问题来了。假设我的data中，code与year相同的记录，其他var有的空值有的非空，那么 在duplicates drop 后，如何实现 后续var非空最多的记录呢？ 也就是最为完整的。  谢谢谢谢！ @蓝色