Duplicate IMEI numbers violate the IMEI Allocation and Approval Guidelines and would lead me to believe those modems are illegitimate clones or have some other issue in their pedigree. IMEI numbers are not intended to be changed by users, only at the time of manufacture, so they are not duplicate because of some needed configuration by users.
Also, since network providers can and do block and blacklist IMEIs of stolen phones, having a whole run with the same number opens you to the chance of having your device blocked (along with what: 100s? 1000s? More? of others) by a blacklist intended to block a single device.
This gives me some sense of concern/alarm about the parts being purchased for these products.