I assume your temp table is a persistent table in your actual system? something like this?

CREATE FUNCTION dbo.MyInlineTableValueFunction()

RETURNS TABLE WITH SCHEMABINDING AS

RETURN

select Distinct UniqueID, RecordingDate

--, ROW_NUMBER() over (Partition by UniqueID Order by RecordingDate)

, CASE When (ROW_NUMBER() over (Partition by UniqueID Order by RecordingDate)) = 1 THEN UniqueID

Else UniqueID + '-' + Convert(varchar(255),ROW_NUMBER() over (Partition by UniqueID Order by RecordingDate)-1)

END UniqueID_Alt

From YourPersistent_Crusty_Table

;

GO