st: Unexpected error from -merge-

[Jacob-Jan Koopmans <jacob-jan.koopmans@student.uva.nl>]

Dear list members,

I am receiving an unexpected error when merging data.
I am using stata 12.

I am trying to merge data in the following way:
"sort actpsid
merge 1:m actpsid using "cbhiind" ".

This, however, results in the following error:
"variable actpsid(my individual identifier) does not uniquely identify
observations in the master data
r(459)."
This would normally suggest(as far as I know), there are duplicates of
actpsid in the master data. However, when checking for duplicates by
using: duplicates report. I receive the following nice report:

"Duplicates in terms of all variables
--------------------------------------------------------
   copies | observations       surplus
----------+--------------------------------------------
        1 |         8772             0
--------------------------------------------------------"

Therefore, I suppose the error is not resulting from duplicates in my
identifier variable.

I would thus like to ask whether there are other reasons why the above
mentioned error appears.

Kind regards,
Jacob-Jan Koopmans
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