Duplicate IPs can be seen depending on how carriers lease IPs from their IP pools. In such cases, you can get access to more IPs by:
Using multiple mobile carriers.
Switching some modems to 3G only which would have different IP addresses. 3G networks address pools are different than 4G. (This option is not recommended as you will get 20-30% lower speeds)
Use signal boosters to tap into neighboring allocations. This step requires experience and not easy to implement.