Marion Kennedy (Campbell) - Are they the same?

Since the two ladies named Marion Campbell married men named John Kennedy, and have the same birth and death dates (probably imaginary) it would appear that they are the same.

John Kennedy in New Galloway and his wife Marion Campbell had a son named David< he was a Merchant in New Galloway. He married Margaret Chapman, died on 15 April 1768, and appears to have been buried beside his wife in Kells Churchyard in Kirkcudbright.

Can I suggest that you read the overviews for David and his father.

It seems unlikely, that that the New Galloway merchant is the same person as David Kennedy, who is said to have married a Seneca woman, died on 15 April 1768, and was buried at Mercerburg in Franklin County, Pennsylvania,

I cannot say for sure, but this appears to be yet another example of the imaginative genealogy so frequently encountered on the internet. It has a tendency to be copied.