Raise error for multiple values in time table for a variable for which this is not supported

[SouthEndMusic]

In one of her models @Fati-Mon put a varying user return factor in the UserDemand / time table because the table allows this. However, this is not supported in the core but it does not raise an error. Therefore:

The core should raise an error if in a time table for a node there is more than 1 unique value over time for a variable for which this is not supported.