Hi Jean,
Yes, I see what you mean. I have come across people entered twice in a census, when away from their usual residence (e.g. in a grandparent's household as well as at home). The 1871 Robert mariner has Marr., from Maughold (could mean Ramsey).
He isn't on the 'Persia' in 1881, unless there was more than one local vessel called that.
From FamilySearch:
Samuel GOODWIN Other Male 39 Plymouth, Devon, England Mate
Thomas DEVONPORT Other Male 30 Manchester, Lancashire, England Ship Keeper
William INGLES Other Male 30 Bannff, Scotland Ship Carpenter
John DENSON Other Male 40 Liverpool, Lancashire, England A B Seaman
James GREGORY Other Male 21 B S, Austria A B Seaman
Source Information:
Vessel "Persia"
Census Place Bootle Cu-m Linacre, Lancashire, England
Family History Library Film 1341885
Public Records Office Reference RG11
Piece / Folio 3694 / 81

These are the Robert Corlett mariners b.IOM found in 1881:
Birthplace Douglas, Isle of Man, Age 69 Occupation Master Mariner
Marital Status W
Source Information:
Dwelling Parkfield Lodge Parkfield Avenue
Census Place Birkenhead, Cheshire, England
Family History Library Film 1341856
Public Records Office Reference RG11
Piece / Folio 3577 / 98
Page Number 31
[He looks as if he's been living in England for 40 years. His son not b.IOM but could have been a visitor? Robert CORLITT Son U Male 39 Birkenhead, Cheshire, England Ship Steward]

Birthplace Douglas, Isle of Man, Age 54
Occupation Ship Carpenter
Marital Status M
Source Information:
Dwelling 13 Mona St
Census Place Onchan, Isle of Man, Isle of Man
Family History Library Film 1342346
Public Records Office Reference RG11
Piece / Folio 5604 / 110
Page Number 13

Birthplace Ballaugh, Isle of Man, Age 36
Occupation Fireman (Seaman)
Marital Status M
Source Information:
Vessel "Snaefell"
Census Place Braddan, Isle of Man
Family History Library Film 1342347
Public Records Office Reference RG11
Piece / Folio 5606 / 30
[Too young if the John you found in the 1861 census was the right one.]

As a mariner Robert could have been in the IOM from anywhere! I think Presentments are the best bet, for the possible years of John's birth.

A number of fathers in marriage certs seem to have been invented, when someone was illegitimate - but how would we know for sure. He looks more likely to be an invention when the child has always lived with the maternal family and has their surname.

Sue