UI allows only a single selection of AggType per MeasureName, but many could be possible

[AnatolyBuga]

eg. I cannot select ("MeasureA", "sum") and ("MeasureA", "count"), but this should be possible unless Aggregation is restricted to only one possible value.

So once "MeasureA" was selected as "sum", it should reappear/remain in the list, and I should be able to find it again and choose "count" or "mean" or whatever