I have never heard or encounter of identical Cell ID (CI) in one Network, even for different RAT from any of the Operators I have worked with.
But I think it is possible to have the same CI on different LAC within the same BSC for as long as these two CI’s do not overlap their coverage, no common Neighbors between them and not having the same BSIC under their homing BSC. The idea is to ensure that no measurement data coming from a Mobile together with the Neighboring cells of its serving Base Station is delivered to the network having two BSIC and CI at the same time. I have never tried or heard that someone tried having two CI’s in one BSC or in one network. Routing of the call from the Core needs CGI where it is composed of MCC + MNC + LAC + CI. So if you have different LAC, CGI will still be unique.
But do you really need to re-use CI in your network? There are 99,999 available CI to choose from.