It means that in either or both of the files you are merging, the School_Code value "2" appears in more than 1 row of the dataset. Each row has to have a unique value for that variable, or the MATCH you specified won't know what to do.

here is a script for removing duplicates.  there is also a point and click & aggregate way to do this.

sort cases by school_code.

if school_code=lag(school_code) drop=1.

select if missing(drop).