This is usually caused by the action being run with Image Processor, not by duplicate filenames themselves. In your case, the recorded action had been overwritten and included steps to close the current file and open another file, so Photoshop followed those recorded commands during the batch.

Check the action you selected in Image Processor and make sure it does not contain any specific Open, Close, or Save steps that point to another file or folder. For batch processing, actions should generally contain only the edits you want applied, while Image Processor handles opening and saving the files.

If needed, re-record the action using only the adjustment steps, then run Image Processor again.