CONCATENATE cells if number in first column is the same

Dear All,
hope you have a simple solution.
I have a few large files containing 2 columns.
Both columns only contain numbers.
If the number in column A2 is the same as A3, i need to concatenate the numbers in B2 and B3 devided by a comma.
Sometimes A2 is the same as A3, A4 etc up to A1000 (and more)

So example

7141 - 1000
7141 - 1001
7141 - 1002
7025 - 1003

I need this outcome : 7141 - 1000,1001,1002

Maybe this is very simple but i can't find a way.

Thanks in advance for looking at it.

Best regards, Ed