Altium - refusing to load footprint
The most likely cause of this issue is that the footprint you made and are trying to add to the existing library already has a footprint of the same unique ID or library reference name. This cannot work, as every footprint must have a unique name. 

I think it is only on a per-library basis; you can have another library of footprints with the same footprint names as the other library, and it will still allow you to use both libraries at the same time.