With a renameCollection command across databases, it is possible that two collections are specified to drop at the same optime, the dropSource and dropTarget collections. When we try to rollback these two drops using 2-phase drop, because the two drops have the same optime, the following error is thrown below. This is only a problem with renameCollection across databases. In a same database renames, only dropTarget can be present.

Failed to add drop-pending collection foo.system.drop.2i0t5.t with drop optime { ts: Timestamp 2000|0, t: 5 }. There is already an existing collection test.system.drop.2i0t5.t with the same drop optime.

The collections are in two different databases, "foo" and "test," but have the same optime, "2i0t5."