Let's say some hypothetical language had the phones [g],[k],[ʔ],[h]
and we determined /g/ to be the UR of the following allophones:
But upon further examination, you may be able to argue for the glottal stop being the UR of another subset of allophones:
/ʔ/: [ʔ], [h]
So based of this definition, the segment ʔ is functioning both as an allophone of the phoneme /g/, and as its own phoneme /ʔ/.
Is this possible?