Define a conditional value list based on the same relationship. (ie. list the values in the value list from Abstract_Property_Join::Proj_Chain_Number and select "include only related values starting from [select your parent table occurrence's name in the drop down]. Then this function call:
ValueLIstITems ( Get ( FIleName ) ; "yourValueListNameInQuotesGoesHere" )
will return the list of unique items.
And if you are using FileMaker 12, you can use ExecuteSQL with the DISTINCT keyword and a "join" expression that recreates your relationship between these two tables to get the exact same list without having to also define a conditional value list.