I use
www.dnsstuff.com to get useful information about IP addresses - in particular, the "IPWHOIS Lookup" will trace the IP. It's true that your customer's IP address will trace to their service provider. You can test this yourself by putting your own IP address into the IPWHOIS Lookup". If you don't know your IP address or where to find it on your PC, the easiest way is to go to
www.whatismyip.com . Then put this IP address into the IPWHOIS Lookup, and it will probably be your ISP.
The thing I would be concerned about is that even though customers may share the ISP, they should each have a unique IP address, unless you have a large volume of customers accessing their accounts from an Internet Cafe. (Yikes!)
How many duplicates are we talking about?
Edited to add: Also, are the IP addresses truly exact duplicates? Or are the last group of numbers different? ISPs own blocks of IP addresses that they assign to their customers, and therefore the numbers will be very similar because they are part of a range of addresses.