When a document moves as a result of an update, we store the new location in
seenObjects. If an update yields and a different update operation moves the
document, the original update could modify it again. I don't know whether or
not this is considered correct behavior (if we changed to tracking _id, there is
another case where this could happen if one update yields and another changes
the _id - though I could see that potentially being considered more desirable).