It appears to be a case of multiple threshold transaction log firings and getting the same transaction log file name by date time stamp up to a second.
Enhance the logic to check if a multiple transaction dumps are queued up in the threshold procedure you use.
If you are on 15+ version try to use big_datetime to formulate your file names.
It may help to add a random number in the file name to make it unique.
HTH
Avinash