Disallow duplicate fields in index validator

[MMcM]

FDBNestedFieldQueryTest.nested2 explicitly creates an index with two adjacent copies of the same field. I do not think that there is any practical benefit to having such an index and that it would most likely arise due a mistake. (On the other hand, duplicate query conditions can arise fairly legitimately with layered tooling and are worth supporting.)

I suggest that rather than getting this test to work in the new planner, we delete it in favor of validator checks that disallow this.