The record of the marriage of my great great great grandparents on this site is as follows:
Marriage of Phillip Hopkin and Sarah

Record_ID: 47091
Entry_Number: 141
Year: 1793
Month: Dec
Day: 2
Grooms_Surname: HOPKIN
Grooms_Forenames: Phillip
Grooms_Age:
Groom_Condition: Bachelor
Grooms_Occupation:
Grooms_Residence: Parish of Woolaston in the County of Gloucester
Grooms_Fathers_Surname:
Grooms_Fathers_Forenames:
Grooms_Fathers_Occupation:
Brides_Surname: HOPKIN
Brides_Forenames: Sarah
Brides_Age:
Brides_Condition: Spinster
Brides_Occupation:
Brides_Residence: Of this Parish
Brides_Fathers_Surname:
Brides_Fathers_Forenames:
Brides_Fathers_Occupation:
Licence_or_Banns: Banns
Date_of_Banns: [not stated]
Signature_or_Mark: Both sign
Witness_1: The mark of William Williams
Witness_2: William Hulin
Other_Witnesses:
Officiating_Minister: Tho[ma]s Edmunds Curate
Event: Marriage
Memoranda:
Notes:
Register_Reference: P278 IN 1/9
Page_Number: 39
Parish_Chapel: St Briavels
Soundex_Groom: H125
Soundex_Bride: H125

I found this a bit difficult to accept - that they had the same surname and come from such a small area. However, I have found a baptism record for a Sarah Hopkin that seems to match.

Do you think I should doubt that her family name is recorded correctly, or is this not unusual and not worth suspicion? Sorry if this is a dumb question...

In a small village, there aren't too many choices when it comes to looking for a girlfriend/boyfriend, so cousins often married. I have several instances in my Family Tree of couples marrying having the same surname. My own G G Grandparents were cousins, one of their daughters married a cousin. It makes followiung descendancies more complicated.

Jim Ashton

OK, thanks Jim. I guess this started to change with the invention of the internal combustion engine...?

Our family out here were disapproving of cousin marriages when I was growing up, so I imagined this attitude was something inherited from the old country - hence my surprise to see cousin marriage suggested by the marriage record of these ancestors of mine.

Thanks again. John

Though they are highly likely to have been related Hopkin/Hopkins is not a rare name and they could have been no relation or very distantly related.
Sometimes people oppose things when things have gone wrong in other generations e.g. a relative marriage may not have worked out which may be why it is opposed in a different generation.
As far as mainstream Christianity and the British law is concerned there is a list of relationships which are allowed and a list not allowed in terms of marriage, I think, but would need to check, that marrying a first cousin is about the closest relative one can legally marry in both Christianity and by law. The C of E (and other mainstream brands of Christianity) in such a close commmunity would only have allowed marriages to take place if they fell within what the Church and the civil law allowed and there would have been harsh penalties for crossing over the well defined lines/"bounds of consanguinity".
Another factor, as well as the very close rural communities in which they lived could be religion e.g. RCs marrying RCs, Quakers marrying Quakers,Jewish people marrying Jewish people, Methodists marrying Methodists, and would therefore restrict their choice further. In the Forest of Dean language group could have been a factor, some people were Welsh speaking and may have therefore married other Welsh speakers or bi-lingual people rather than those who spoke English as their only language, which would restrict choice further.
Depending on the era in which people lived there would also be an element of arranged marriage, not necessarily done formally but by community expectation and pressure from those around them to marry a certain person or someone from the same class. Certainly formally arranged marriages were very common in Tudor times but also at other times too.
In my family there are examples of people marrying relatives, not necessarily as close as first cousins, e.g. relatives of step relatives marrying; marrying the brother of the mother's second husband (which made her mother also her sister-in-law and her step-father was therefore her brother-in-law, her half-sibings were also her nieces/nephew through marriage), etc.
I don't know where you are living now but if you are abroad, you might be interested to know that if any of your family were from over the border in Wales that the number of surnames there was relatively small. It is therefore quite common for a Jones to marry a Jones; or Mr. Evans marrying Miss Evans and a Morgan and a Morgan without them being related (or only related by many distant generations). I have found looking up Welsh relatives with common Welsh names is challenging for this reason.
Also in Wales it was traditional to call people by their father's name, e.g. Evan Ap Evans reduced now to Evan Evans, or Thomas son of Thomas being Thomas Thomas, Owen Owen, Morgan Morgan etc.

Looking at this discussion I would like to make a couple of points.

Marriage between first cousins was legalized in the 1540s.

Before the industrial revolution the majority of people were born, lived, worked and died within fifteen miles of their birthplace. At this time it has been estimated that two to three percent of marriages were between first cousins.

The marriage may well have been arranged in order to keep property, etc. within the family.

On the other hand an error by the clerk is not unknown. The marriage record states that both the groom and bride signed. Checking the original record should confirm the names.

Thank you to both rookancestrybest and Paul for lots of good points, explanations etc; all well made.

Yes, I live as far away from the Forest as it is possible to be (short of Antarctica) - the South Island of NZ. Your comments reminded me of four things:

1. my great grandfather was Welsh speaking, so it is highly probable that his grandparents were as well, thus potentially reducing the pool of marriage candidates.

2. I discussed this with my sister this morning because I remember our family sabotaging a cousin to cousin relationship. Whilst it was said to have been done because of the cousin issue, we both suspect that the real reason was because the young woman's birth was outside of marriage. Her parentage was a secret kept from her father's subsequent wife and son (her half brother) - she was however recognised by our grandparents and others in the family! If the two cousins had got closer they would have found out they shared the same grandparents and the secret would have got out. Stupid family secrets...

3. I also recall a Jewish student of mine, who told me she was migrating to Australia because the Jewish community here was so small she thought she would never be able to find a husband - all the Jewish young men here were like brothers to her and it would have felt like incest to marry any of them.

4. Finally, a friend comes from a farming family in a remote South Island community. All of the farms in the district were once owned by relations, but are now dispersed. Marriages in those days happened between people who were less than a horse ride or walk away, but subsequently marriages happened within a much wider district and thus the farms were eventually lost. As his cousin said "the internal combustion engine has a great deal to answer for".

Thank you Paul for the suggestion of checking the signatures for final proof.