Swift control flow: Unwrapping nil despite if check?

Note that the types of internalId and orderDate are double optionals - String?? and Date?? respectively. This happens because resultDict["internalId"] produces a Any? (the key could be absent in the dictionary), and the safe cast to String? produces a String??, as there are 3 cases:

• the key is present in the dictionary, and the cast succeeds (both layers of optional would be non-nil)
• the key is absent in the dictionary (the outer optional would be non-nil, wrapping a nil as the inner optional)
• the cast fails (the outer optional is nil)

== nil only checks if the outermost layer of the optional is nil, but you are unwrapping both layers with two exclamation marks. The first exclamation mark is safe as ensured by your if statement, but the second is not.

You can avoid all of this if you just cast to String and Date instead:

let internalId = resultDict["internalId"] as? String
let orderDate = resultDict["orderDate"] as? Date

You can also use a guard statement to eliminate the forced unwrapping:

guard let internalId = resultDict["internalId"] as? String,
      let orderDate = resultDict["orderDate"] as? Date else {
    #if DEBUG
    fatalError("internalId/orderDate was nil")
    #endif
    continue
}

do {
    try removeAllButOne(internalId: internalId, orderDate: orderDate)
} catch {
    LOGGER.error("Could not deduplicate.")
}