Finding Duplicates - Then finding delta

Trying to find out if something exists that performs the following:

I have a master spreadsheet where as the key (unique) data exists in column A
Currently there are over 1000 rows of data in all
At the end of each month I have to add data to this worksheet.
Note that data is pulled from another application, provided in a worksheet (child worksheet) and then is pasted into the master spreadsheet

But there is the possibility that the unique value in column A can be found in both spreadsheets.
So I want to be able to find and show only the duplicates.
This way I can compare the row of data for each spreadsheet and if they are different, put the updated data from the child spreadsheet into the master spreadsheet.

I hope I explained this well enough.