Yes, CAS and Interdiction have some overlap. It all depends upon how "deep" a ground commander is able to see or think, and at what echelon he operates at.
To be pedantic, there is no overlap between CAS and Air Interdiction in current US doctrine. A battlefield support mission is CAS if it requires detailed integration with the fire and movement of friendly forces. If it doesn't require detailed integration then it's Air Interdiction. Typically the split between the two types is denoted by the FSCL, but other control means can be used to temporarily remove the need for integration.
It might be more accurate to say that most attack aircraft overlap between CAS and Air Interdiction roles, because if you can do one, you can do the other!