I have a dataset in which the data are identified by state, county, and ZIP code - each of these is a different variable.

My problem is that some ZIP codes span over more than one county, so I need to be able to identfy which observation of the variable "ZIP: have different values on the variable "county". Fow example, below, I need to be able to identify that 00112 is a ZIP that has different values (1 and 2) on the variable "county", because it spans across two counties:

County - ZIP
1 - 00111
1 - 00112
2 - 00112
2 - 00113
2 - 00114

What code would allow me to do that for the entire dataset? Thank you in advance!