Login Script - a log file overwritten by echo %var% >> file

A cmd file is being called from the login script. If a
condition is met the computer name is written to a log
file housed on a share. Users have rights to write to the
file but not delete. The problem is randomly (once or
twice a week) the log file is being overwritten. An
example of the syntax in the cmd file is below:

echo %COMPUTERNAME% >> \\servername\share\logfile

ideas?

thanks