Description
Coincident data with different values indicates measurement error. For semivariograms, the nugget is composed of microstructure and measurement error. Thus, if the dataset contains two or more different values for a single point, some measurement error exists, and the nugget cannot be zero.
Solution
Increase the nugget value of the semivariogram model so measurement error can be included. Alternatively, choose a method to handle coincident values (for example, use the mean value).