I'm not sure what you're getting at here.

For example, if you assign piano to a specific channel, then you can play piano perfectly normally on that channel including chords, so you can have any number of piano notes sounding at the same time. This is perfectly normal. In the same way, if you assign flute to a channel and it's an orchestral piece which maybe represents a flute section of an orchestra then you might have multiple flutes. Clearly you need to watch the capacity of your midi device as to how many separate notes you can play.

If you want to play the same note multiple times then this may well be dependant on the device, and how it handles multiple versions of the same note. Some devices may be able to play only one version of a specific note at the same time on any one channel.

If you want the correct sonic effect of two instruments playing the same thing, then you would really need to use two separate channels, with say a slight difference in the pan setting, to give some separation.

Trying to use different velocity would not help if the midi instrument will not play the same note more than once.

As regards your specific query, no, no midi instrument would modify the sound as you suggest, as far as I'm aware. The device would merely follow the instructions it's been sent.

Part of the answer here relates to the provision of various 'ensemble' sounds, where the actual sound has been altered slightly to give a difference between a solo instrument and multiple instruments. Such an option is not available for all instruments however. A chorus effect would also help, but again this would affect everything on a specific channel, however 'chorus' is intended to swell out the sound to maks a single 'voice' sound more like a number of 'voices'.

If you're using a specific device, you shoulkd create a test midi file that does play the same note multiple times at once, and listen if you can hear any difference, and try various changes to see if a difference becomes audible - but what would on one device may not do the same on another?

Geoff

This is not specified.

The second note-on message might cancel the first note, or might start a second sound, independet of the first one. In the latter case, nobody agrees on how the note-off messages should be handled.