ResourceAssignmentColumn may duplicate same resource

[matsbryntse]

Reported here: https://www.bryntum.com/forum/viewtopic.php?f=52&t=12273

From the post:

This issue is really not easy to reproduce constantly.
As a matter of fact, once we had the issue, we tried to reproduce it and, the issue occurs 3 times of out 10 try.
Then, we are explaining to you herewith how we did have that issue :

1. From your online demo (https://www.bryntum.com/examples/gantt/advanced/)
2. Select from the Grid a Task without assignment.
3. Click the Cell and Add one Resources, BUT do not touch the units.
4. Save and click anywhere to render clear the focus on the selected Cell
5. Go back to the Cell and Click to open up the Assignment editor the ONLY change the Units and Press Enter.
6. Then Click Save

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I tested it and reproduced the issue on the first try.

It may be related to this ticket: https://app.assembla.com/spaces/bryntum/tickets/9284-resourceasignmentcolumn-units-can-get-out-of-sync-with-data-in-some-circumstan/details

[ExtAnimal]

https://github.com/bryntum/bryntum-suite/pull/622