Such cases may happen due to a number of reasons.
The IMEI number can be duplicated on more than one device. This happens with non-type approved mobile phone devices manufactured by small, non-mainstream manufacturers. However, the percentage of such cases worldwide is low in relation to the total number of mobile phone devices in existence.
Another user may have registered it as belonging to him by accident.
It is registered by a previous owner of a device that has since been sold off or given to somebody else.
In these cases, the www.blockmyphone.my portal would promptly inform the registrant that the IMEI number/mobile phone device in question has been registered by another user.
The notification is purely for informational purposes and the registrant may still proceed with the registration of that device.