"Raw value for enum case is not unique" for Swift enum with Float raw values

It definitely says you can, but don't use floating-point values (and especially Float) where you're going to need to compare equality -- the precision just isn't guaranteed to be good enough. And always use Double unless you absolutely need to use a Float for compatibility reasons.

In this case, it seems like it's having trouble because (a) the third case is 2x the first case, and (b) some other factor I don't know. Using 3.3/6.6, 3.4/6.8, and 3.6/7.2 also gave me the issue, but 3.5/7.0 did not. However, I could get it to show up by changing the last case to 22.2 (2x 11.1).

Here's a workaround — use a typical Int-based enumeration, and provide a doubleValue property:

enum BatteryVoltage: Int {
    case v3v7
    case v5v0
    case v7v4
    case v11v1
    case v12v0

    var doubleValue: Double {
        switch self {
        case .v3v7: return 3.7
        case .v5v0: return 5.0
        case .v7v4: return 7.4
        case .v11v1: return 11.1
        case .v12v0: return 12.0
        }
    }
}

There are some nice additional features of enums you can take advantage of if they're Int-based.

(From my above comment:)

That looks definitely like a bug. It seems to happen if one enum value is exactly equal to "2 times another enum value", but not equal to an integer.

More generally (as @Sulthan observed) the error occurs if the ratio of the enumeration values is a power of two, such as 3.7/7.4, 1.2/4.8, or 1.1/17.6, but only if both values have a non-zero fractional part. So 1.5/3.0 or 1.25/5.0 do not cause an error.