If you select Port 20 and it assigns a synth, then you select another track and assign it to port 20 it does not create another instance of the synth here.

Actually, the Port number in the drop down to select it now should show the synth loaded in that slot (not the port number anymore, see image below)
Can you post screen shots of how you are doing this?

Usually this problem is solved by making sure you have a patch assigned to that MIDI channel in the synth.
Many tracks can use one port (the same synth), but the synth needs to be capable and set up to handle the MIDI channels those tracks are sending it.
Assigning a MIDI channel is separate from Port.

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