Curiously she has two baptisms on file, perhaps the first was forgotten.  Anyhow the first was in 1798 and the image of the record reads:

“Mary Daughter of Henry Duncan Junr Esqr a Captain in the Royal Navy & Mary his wife, bapt July 3”

And the FindMyPast transcription is:

First name(s)           Mary
Last name               Duncan Junr
Birth year              -
Baptism year            1798
Baptism date            03 Jul 1798
Denomination            Anglican
Baptism place           Stoke Fleming
Father's first name(s)  Henry
Mother's first name(s)  Mary
County                  Devon
Archive                 South West Heritage Trust
Archive reference       1342A/PR/1/4
Record set              Devon Baptisms

The second baptism was in 1815 and collected by FamilySearch and it gives an ‘original’ birth date plus a second forename:

Name:                    Mary Anna Duncan
Event Type:              Christening
Event Date:              25 Jan 1815
Event Place:             Dartmouth, Devon, England, United Kingdom
Event Place (Original):  Townstal or St Clement, Dartmouth, Devon, England
Gender:                  Female
Birth Date:              7 Jun 1798
Father's Name:           Henry Duncan
Mother's Name:           Mary

But between all that there is enough information to substantiate that she was the daughter of Capt Henry Duncan.
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I have not found her in the 1841 or 1861.  The 1841 census would probably have shown their children but I could not search be her birth county as the 1841 does record that unless they still lived in their birth county.  But I have now, with her birth given as in 1806:

They were at Ivy Tower, St Florence, Pembroke, Pembrokeshire, Wales

First name(s)  Last name  Sex  Age  Birth year  Birth place
F P M      Myers      Male    35  1806  -
Mary Anna  Myers      Female  35  1806  -
Frederick  Myers      Male     8  1833  Pembrokeshire, Wales
William    Myers      Male     6  1835  Pembrokeshire, Wales
Fanny      Myers      Female   4  1837  Pembrokeshire, Wales
Arthur     Myers      Male     2  1839  Pembrokeshire, Wales
Mary       Duncan     Female  60  1781  -
Isabella   Duncan     Female  35  1806  -
Dorothy    Lewis      Female  25  1816  Pembrokeshire, Wales
Mary       Jenkins    Female  25  1816  Pembrokeshire, Wales
Sarah      Harries    Female  20  1821  Pembrokeshire, Wales
Elizabeth  Gwyther    Female  20  1821  Pembrokeshire, Wales
Martha     Morgan     Female  15  1826  Pembrokeshire, Wales
Betsy      Griffiths  Female  25  1816  Pembrokeshire, Wales
Robert     Lewis      Male    20  1821  Pembrokeshire, Wales

Ref:
Archive reference  HO107
Piece number       1443
Book number        5
Folio number       15
Page number        13
Combined:          HO107, piece 1443, book 5, folio 15. p. 13

His name is celarly FPM Myers on the enumerator’s sheet.
The Isabella Duncan above was mary Anna’s sister, who signed the marriage register for Mary Anna.  Isabella’s death is not to be found
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In 1851 she and her husband had her mother with her in at Croft Terrace, Saint Mary Tenby in Liberty, Pembroke, Pembrokeshire, Wales:

First name(s)  Last name  Relationship  Marital status  Sex  Age  Birth year  Occupation  Birth place

John P M Myers       Head           Married    Male    49  1802  Landed Proprietor And Miner   Cheltenham, Gloucestershire
Mary A Myers         Wife           Married    Female  49  1802  -             Stoke, Devon, England
Mary Dincan          Mother In Law  Widow      Female  76  1775  Of The Late Capt H Duncan R N    Plymouth, Devon, England
Elizabeth Nash       Servant        Unmarried  Female  37  1814  House Serv    Pembrokeshire, Wales
Eliza Morris         Servant        Unmarried  Female  30  1821  House Serv    Pembrokeshire, Wales
Elizabeth James      Servant        Unmarried  Female  28  1823  House Serv    Pembrokeshire, Wales
Martha Bowen         Servant        Unmarried  Female  27  1824  House Serv    Pembrokeshire, Wales
Alexander Griffiths  Servant        Unmarried  Male    22  1829  House Serv    Pembrokeshire, Wales

Ref:
Archive reference  HO107
Piece number       2476
Folio              109
Page                45
Combined:          HO107, piece 2476, folio 109, p. 45

I wonder if Mary Anna had concealed from her husband that she was three or so years older than him?

Her mother definitely has her name spelled Dincan on the enumerator’s sheet.
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In the 1871 census she was a lodger at Palace Square, Croydon, Surrey:

First name(s)  Last name  Relationship  Marital status  Sex  Age  Birth year  Occupation  Birth place
Mary Ann Myers Lodger - Female 72 1799 - Devon

Archive reference  RG10
Piece number       850
Folio              42
Page               77
Combined:          RG10 piece 850, folio 42, p. 77

The enumerator’s sheet records her birth as in Stoke Damerel, Devon, so I’ve added that to the above.  Her birth year corroborates better with her baptism records.
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It looks like all her children were born in Pembrokeshire and that they moved around a bit, from these baptisms in the Pembrokeshire records:

First name(s)  Last name  Birth year  Baptism year  Father's first name(s)  Mother's first name(s)  Place

Mary Ann               Myers  1827   1827          John                    Mary    Haverfordwest, St Martin
John Duncan            Myers  1829   1829          John Powell Matthew     Mary Anna           Tenby
Henry Duncan           Myers  1831   1831          John Powell             Mary Anna           Prendergast
Frederic William       Myers  1833   1833          John Powell Matthew     Mary Anna           Prendergast
William                Myers  1835   1835          John Powell Matthew     Mary Anna           St Florence
Fanny Stephenson       Myers  1836   1836          John Matthew Powell     Mary Anna           St Florence
Arthur Bowen Richards  Myers  1838   1838          John Towell Matthew     Mary Anna           St Florence
Isabella Duncan        Myers  1841   1841          John Powell Matthew     Mary Anna           St Florence
Duncan                 Myers  1844   1844          John Powell Matthew     Mary ASnna          St Florence

Fanny and Isabella both married in India in 1861 and 1862 respectively.  Isabella Duncan Lloyd died in 1871 in Clifton, Bristol in the first quarter so was not in the 1871 census; probate was given to her husband Commander Charles Lloyd, Indian Nay, she died on 19 March 1871 at 23 The Mall Clifton.
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This is the only Mary Anna death index: from 1872 to 1876:

First name(s)  MARY ANNA
Last name      MYERS
Gender         Female
Birth year     1799
Age            75
Death quarter  4
Death year     1874
District       Windsor
County         Berkshire
Volume         2C
Page           274
Country        England
Record set     England & Wales Deaths 1837-2007

It is confirmed to be her by the naming in the probate index of her son, Arthur Bowen Richards, and the death date given for probate of 20 October 1874.
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Her probable probate index:

1874
21 November
The Will of Mary Anna Myers formerly of 1 Valley-view Clevedon in the County of Somerset but late of 11 High-Street New Windsor in the County of Berks Widow who died 20 October 1874 at 11 High-Street was proved
at the Principal Registry by Arthur Bowen Richards Myers of Coldstream Guards’ Hospital Vincent-square Westminster in the County of Middlesex Surgeon in the Coldstream Guards the Son and Charles Lloyd of 11 High-Street Commander in Her Majesty’s late Indian navy the Executor.
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The marriage abstract on FamilySearch:

Name:                    John Powell Matthew Myers
Event Type:              Marriage
Event Date:              6 Dec 1826
Event Place:             Brighton, Sussex, England, United Kingdom
Event Place (Original):  St. Nicholas, Brighton, Sussex, England
Gender:                  Male
Marital Status:          Single
Spouse's Name:           Mary Ann Duncan
Spouse's Gender:         Female
Spouse's Marital Status: Single
Record Number:           295

And this was found on page 555 of the film of the Registers for this charch.

This image added:

His Condition:        Bachelor
Her Condition:        Spinster
Celebrant:            J S M Anderson, curate
Witnesses:            Isabella Duncan, Frederick Smyth, Mary Duncan

Of the witnesses, the first and the last were Mary Anna’s sister and mother respectively, but Frederick Smyth is not known.
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