Portal filtering only on duplicate field values
I am really new to FMP, but am setting up a database from scratch to contain my lab project data and I cannot figure out how to set up a portal filter to display only records with duplicate values in one particular field. I have been able to successfully set up portal filters to display based on other types of field information but this one has me stumped. I am using FMP 11- any insight would really help me. I know I'm missing something very basic- it must be that I cannot come up with the appropriate calculation I need. I appreciate any help in advance!