FTP rename results in two files in IFS

We are running a third-party voice picking application on a win-tel server
on our network.

When orders are ready for the warehouse to load, we send inventory picking
info to the Voice Pick
server by putting our iSeries data in .csv files.

As voice picking results become available on the VP server, the results
are put in a .csv file and
sent by FTP to a directory under the IFS Root on our iSeries,
resulting in hundreds of small files a day.

We run a process every few minutes to read these .csv files and post
picking results files on our iSeries

To avoid processing a file before the FTP process completes, we use a
naming convention where
the voice pick server does the FTP PUT using a file named with a
prefix of FTP_, and the iSeries
post process ignores any such files.

When the PUT command completes, the VP server re-names the file, removing
the prefix.

About 98% of the time (just a guess) the re-name works as expected.

For some files, however, the re-name results in two files, the "first"
file that has the FTP_ prefix and the
"second" or re-named file with-out the prefix.

Has anyone seen this?

Any suggestions as to a fix?