Hi. Thanks for getting in touch.
As you have seen, duplicate ordinals in DataPoints are not permitted when one sets a stackId on a series. Whilst we generally make no restrictions on duplicate DataPoint values, it is necessary for the principle of stacked series. We have not exposed any way in which we can filter any duplicate data values, we have placed that responsibility on to the user. To overcome this issue a suggestion would be to strip out duplicate data values at the earliest opportunity. This might be at the point of generating the data itself but of course you may have no control over your data, especially if it is coming from an external source. In this case you might like to implement your own DataAdapter, and add some custom logic which filters out duplicate data values before the creation and addition of DataPoints.
I hope this helps, thanks,