His Highness the Sovereign Prince passed a decree outlawing dual citizenship with other states, with a few exceptions made. This decree is most likely the outcome of sharing citizens with many other micronations, a common problem in micronationality. While a Princely Decree, it comes as a request of the Princely Cabinet, as citizenship can only be dealt with through decrees.
As such combining the Montescanesque citizenship with that of other entities is banned, with the exception of:
Montescano furthermore banned citizens from being a member of numerous criminal and/or terrorist organizations, and established provisions for the automatic removal of citizenship of members joining those.
The full decree (dutch) can be read here.