In version 2.5.3, we introduced a new format for file names of forms (.mxf files) as part of the improvement that you no longer need to clear your browser cache when you redeploy from the Modeler. As a result of this, forms get a new file name every time you deploy from the Modeler.
At this moment, old forms are not cleaned up. However, you can clean up your deployment directory by choosing Project > Clean deployment directory from the Modeler's menu bar.
Note that if you create a deployment archive (.mda file), the deployment directory is automatically cleaned first, and only the latest form files are put in the archive.
If you think this is a bug, and Mendix should clean the old form files upon each deployment, feel free to add a ticket in our support portal.