When specifying both an output file with -o and output format with -f, dmtxwrite currently throws a warning stating that the user may have requested conflicting formats:
dmtxwrite -v -f pdf -otest.pdf atc12345.txt
WARNING: --format (-f) argument ignored; Format taken from filename
However, as seen in the example, dmtxwrite was forced to choose between ".pdf" and ".pdf" which isn't a conflict at all. Perhaps dmtxwrite should only throw the warning if there is, in fact, a conflict.