Several answers, at least

They were not - it just appears that way in Newland and Bream for instance http://www.forum.forest-of-dean.net/index.php?id=20848

They were - one as a child , one later perhaps as a non-conformist convert

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Ἀριστοτέλης A Gloster & Hereford Boy in the Forest of Dean ><((((*>

In my tree I have the TROTTER family who were christened in St Mary's, Birmingham. Where their father was from. Later I think after he died they were Baptised in Coleford Baptist.

From CoE to Baptist.

David

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Rhondda Valley Boy

Sometimes if a newborn child was very ill they were baptised at home (seen as baptised privately) as soon as possible after birth & then if they survived they were "welcomed into the church" which can be seen as a second baptism. From family comments I have found that it was not always recorded as 1) private & then 2) welomed etc.
Hope this is of some help,
with regards, auntie Ma

I endorse what the others have said here. I am not very religious myself but have some knowledge of the subject. Either a home baptism if a child was at risk of dying followed by a formal one, or a change in religious persuasion. The reason why anyone could baptise a child in danger of dying is because of the belief that without being baptised a child's soul would not go to Heaven but would be in a state of Limbo, a "place" or state of existence between Heaven and Earth but not purgatory. This is because the belief is people are in a state of "original sin" until baptised.
Bear in mind too that some non-conformist denominations engage(d) in adult baptism when the person is able to consent to it, not just the Baptist Church/United Reformed Church but others too. So, if someone was baptised in the C of E and then became a Baptist they might be baptised once as a child and once as an adult.
Generally the RC and C of E recognise each other's baptisms and have done for some time but that would not exclude someone being baptised in both of these either. (I think there has been an earlier posting on this but I might have seen it elsewhere).
Baptism is about being welcomed into a Church and an expression of faith and some churches baptise(d) people as a reaffirmation. I used to live in the USA and was present at a baptism in a lake of an adult being baptised even though he had already been baptised as a child. He did it because he was joining his wife's church.
From recently looking at the Mormons/Church of Jesus Christ of Latter Day Saints my understanding of it is that this denomination actually baptises people after they have died as well as when they are alive. In some cases decades after people died, as well as when they are alive and they have, at least in the past, baptised people as Mormons even though they have previously been baptised into another Christian denomination when people have joined their Church. But correct me if I am not accurate here!

Way back in my family history (not the Forest of Dean line) I have some evidence of someone possibly being baptised twice under two different Tudor monarchs (Henry VIII and Queen Mary) and my assumption is that it was due to the Reformation and the switch back and forth from C of E to R.C.