OTO's 76 mm gun and claims of originality

Do copying accusations reflect a belief that some countries monopolise good ideas?

Sometimes, but a documented lineage is also mistaken for proof that all comparable weapons are copies.

A traceable lineage can encourage people to regard one weapon as the original and all similar designs as derivative. This example follows Vickers three-inch guns through an Italian partnership and later OTO development. That does not by itself establish copying by another manufacturer. Three-inch gun information is widely available and a development team would reasonably study it.