Unable to add Contacts in List (Segments & Lists)

Hi,

I have a list created in Segments in List section of Directory.

I have no rules for merging contacts.

I already have a record in list as below (for example):

I now would like to add another record as below (Only ExternalDataReference is new):

But it updates the 1st record and shows only 1 record as below updating the ExternalDataReference:

I have other lists exactly same as this with also no rules set and it add them as 2 records.

Please suggest what am I doing wrong.

Thanks.