If after uploading Vantiv residuals you are taken to the page which lists new merchants that will be added, but such merchants are already in IRIS CRM, it is possible that the MID numbers in the file you imported got rounded, thus causing the merchants to be shown as new / non-existing merchants.
Open your file and see if the MID numbers are shortened or rounded. If all accounts look like they have the same short MID, then that indicates the file was opened in Excel and saved after it was downloaded originally (which caused the numbers to be rounded due to a built-in behavior in MS Excel).
Note that in Vantiv residuals files each account has multiple rows, so it's likely that the first several rows will have the same MID, and then the next several rows will have the same new MID, and so on. This report setup is expected and correct.
Although the Vantiv MID numbers can vary, a typical Vantiv MID number looks like this: 520001234567.
At first, Excel may show the MID numbers (in the MERCH_NBR column) in the scientific notation (e.g. 5.2E+11). If that it the case and if you try to re-format the MERCH_NBR column cells to the Number formatting, your MID numbers will look normal but they may still not be unique (in case any rounding had occurred).
If the file has all the MID's shortened and the full numbers cannot be shown, the file will need to be downloaded from Vantiv again and re-sent to IRIS CRM support (this time without opening and re-saving the file). IRIS CRM uses only the MerchDetail file for each month.