From what I understand, IMEI really isn’t used for much these days in modern signalling protocols (and even OS anti-theft logic), precisely because it’s been left up to manufacturers to allocate IMEIs, and some manufacturers are dumb and make mistakes like this.

For MAC addresses (also mfgr-allocated) mistakes like this are “fine”, because MAC addresses are layer-2 (switch-local) and therefore a collision will translate to a direct, customer-visible problem: they’ll buy two of the same device, and then their LAN will start acting wonky. As such, market forces will address dumb Ethernet equipment mfgrs, pushing them out of the market as customers stop buying them.

But the people buying cellular handsets aren’t the same people running the network, so there are no market forces in play to push back against dumb handset mfgrs. Instead, the network operators must just play with the hand they’re dealt; and that means coming up with alternative strategies for managing handset connectivity that don’t involve the theoretical primary key (IMEI) but instead make use of other unique keys (e.g. IMSI) that are also included in the connection handshake.