I discovered a match on ancestry.com that was a first cousin match to me on my fathers side but assumed it was his half sibling (we suspected existed.) We share 756 centimorgans. He is five years older and they share a strong resemblance, almost identical. But I have discovered my known second cousin also shares 749 centimorgans with him. Our other second cousins do not share this close a relationship with him. We know my father and his half brother share a mother. How can my fathers first cousin's daughter share this amount of DNA with him as well. We also have many family skeletons but how is this possible?
I note that your matches are on Ancestry.com.
Have any of the three of you (yourself, your half uncle, and your second cousin with the stronger match) uploaded your data to gedmatch?
There, you would be able to visualize the locations of your matching DNA segments.
Are any of the other second cousins you mention siblings to the one with the stronger match?
I'll refer to said stronger match as A from here on out. If none of the others share both presumed parents with A, it is possible that both of A's parents are related to your half uncle's parents, resulting in a higher cM match than you would anticipate from the calculated 1/2 great uncle relationship alone. I'm assuming here that A is descended from a sibling of your father's mother.
Is that correct?