seems like a issue of BlackBerry. Key UP on hardware keyboard is bound to dosbox’s key UP, the same is for key DOWN. Maybe your hardware keyboard does DOWN and UP immediately, I saw it few times in the past, but it is not common. Do you map keys in magic dosbox’s voodoo control/ mapper menu?
Virtual keyboard is special and depends on developer, include all settings, there are keyboards like hacker’s keyboard which is very near to desktop hardware keyboard. This is reason why magic dosbox has own virtual keyboard, we have full control and we can fully control dosbox.
Do test: Create new game profile and start to black console. Press key ‘a’ on your hardware keyboard and hold it, what happens? Is this letter duplicated in command line?