Simple macro for data cleaning, help appreciated

I have an excel file containing following data:

ID , Amount , Year , Company

1 , 1000 , 2005 , ABC
2 , 1000 , 2005 , CDF
3 , 1000 , 2005 , ABC
3 , 500 , 2005 , CDF
4 , 500 , 2005 , ABC
4 , 1000 , 2006 , ABC

Now I want to add cell B3 and B4 and merge both lines 3 and 4, same for ID=4 at line 5 and 6, but want to keep the ID and rest of the information intact in line 3 for ID=3 and in line 4 for ID=4. Number of line item with same ID may vary. There are 12344 lines in this sheet :( . I have another sheet which contains same IDs as in aforesaid sheet and respective customer names. I am supposed to merge both the sheets eventually, which is easy. However, I failed to come up with a solutions for the problem I just mentioned. I need your much appreciated help. Thanks in advance.
Regards

Saya