error merge m:1 no? unique numbers in using data

[JJ vanK]

Hi there,
I am trying to merge 2 datasets with m:1, on keyvar ID.
i am getting an error that "variable ID does not uniquely identify observations in the using data".
When I look at my ID variable in the using data, there are only unique ID-values. when I check with "tab ID' I also see unique numbers.
Am I missing something?
gr Jessy

[Marcos Almeida]

Instead of tabulating, you may check the "using" data for duplicates. Type - help duplicates - and check whether this helps.

[Mike Lacy]

One guess here would be that you have multiple missing values for ID in the using data file, which won't show up in -tab- unless specifically requested.