Filter Out Unique Records

I have a very basic table of customers. In one column there is duplicate data. I guess in most cases an Excel user would only want to filter the table to show the unique records only. In my case I want to do it the opposite way round, to delete the unique records so I have multiple occurrences of strings that appear in that one column. I've used the "Conditional Formatting" trick, which is great = COUNTIF($G$1:$G$44000,G1)>1 highlights all of the strings that appear more than once. What I'd like to do from there though is to either just have that data, and to remove the unique records.

Either that, or.. is there some way to have a field/column which shows "True" or "False" if such a string has appeared more than once in a column. Auto Merged Post;I forgot to mention.. the reason I'd want a column of "True" and False" would be because then I'd have the ability to sort/group the data into all the recurring records and all the unique ones. I'd then be able to do away with the unique ones by just copying the recurring ones.