Try using the very handy '1901 England & Wales Census Decoder' available to download free online..
I have just done a quick search on it, and have come up with the following possibles:
1901
At one address:
Mary MORGAN 38 b c1863 Glamorgan Aberdare, census Glamorgan Aberdare
Mary MORGAN 16 b c1885 Glamorgan Aberdare, census Glamorgan Aberdare
and at another:
Mary MORGAN 46 b c1855 Glamorgan Aberdare, census Glamorgan Aberdare
Mary MORGAN 18 b c1883 Glamorgan Aberdare, census Glamorgan Aberdare
I have only done a quick search, looking for the census matches where there are two Mary MORGANs at the same page reference. I have also specified Aberdare as the place of birth for both individuals.
Download the census decoder...
http://www.censusdecoder.com/It is extremely valuable in pinpointing the exact census page to pay for from the PRO site.