When the indexing is done it is sometimes done in some rather peculiar ways. Here is an example from a ships passenger list of what I mean:
Rather than the indexer seeing this as one family with the surname Schmidt they indexed it this way:
No, I'm not kidding. Rather than see Maria or Peter as a middle name they took it as the person's surname and assumed it was also the surname of the persons listed below. So far I have literally found hundreds indexed like this.
Many are easy to find as they will often be listed with the surname like Maria, Peter, etc. I have found a ton listed with the first name of Maria and a surname also of Maria. Is it possible that in the above example there are three surnames involved and that there were lots of women named Maria Maria? Of course, but I wouldn't bet on it.