Macro to concatenate cells adjacent to duplicates

Sample.xlsxI should prefix this post by acknowledging that I have viewed the other topics with this theme. Mine is slightly unique although I'll put a link to these older topics for your convenience. Below I have what I would like my table to display. In the first too columns we have two ID's. Ideally what I would like to do is concatinate the ID's of (ID2) in the concatID if they share duplicate (ID1) values Without deleting any rows. Therefore the ConcatID column will contain multiple repeated concatinated values due too there being numerous ID2's sharing duplicate ID1's.
_______ ID1 _________ ID2 _______________ CONCATID
111001-0401. 111001-0401 . 111001-0401, 121001-0401
111001-0401 .121001-0401 . 111001-0401, 121001-0401
121001-0402. 121001-0402 . 121001-0402
121001-0403. 121001-0403 . 121001-0403

In other words I want to do this: http://www.excelforum.com/excel-prog...uplicates.html
but without deleting any rows. So in contrast I want to concatenate values based on ID(1) but have this concatenation occur identically in each row where they might be any duplicates.

Too do so I know I have to create a VBA code like this: except without the deleting the rows part.
(All credit for the code from JBeaucaire) Here is another forum post doing what I want to do exept the part about deleting the rows http://www.excelforum.com/excel-prog...nd-delete.html

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Hopefully this post wasn't too confusing. Let me know if I can be of assistance. I'm new here so I would appreciate any help you can offer.
Thanks so much!
(edit) Provided attachment as my table format was messed up.