Grouping a Table based on Text [on hold]

I've used subtotals, group by, pivot tables, etc in Excel to sum columns based on an ID. I get tables in the 100's of rows each week with an ID column with duplicate values like:

table:  
ID  Location
a   Boston
a   NY
c   SF
d   LA
c   Seattle

I want to be able to group them without losing data:

output: 
ID  Location
a   Boston, NY
c   SF, Seattle
d   LA

How would I do this?

Data Sample #2 (using @pnuts formulas) - the problem here is that after deleting the TRUE rows, banner1 does not show the 1. It should be 1,2,5 in the formula1 column in the banner1 FALSE row:

id             cell formula1    formula2
banner1        1    1,          TRUE
banner1        2    2,          TRUE
banner1        5    2, 5        FALSE
banner2        3    3,          TRUE
banner2        6    3, 6        FALSE
banner4        4    4,          TRUE
banner4        7    4, 7        FALSE