Aspell creates backup files with a .bak extension by default. To turn off the backup files configure Aspell to not create them. Often there is not a not a user configuration file “aspell.conf” present. Even if there is a config file present, it can be overridden by environment variable ASPELL_CONF:

export ASPELL_CONF="dont-backup;"

or do similarly through Control Panel in Windows.

Confirm the setting has taken effect by:

aspell dump config | more

and look for the line backup false.

Related: Aspell dictionary location