var arr = [String]()
if !arr.contains(element) {
      arr.append(element)
}

    var arr = [String]()
    if !arr.contains(element) {
          arr.append(element)
    }

or 
extension Array where Element: Equatable {
    mutating func appendIfNotExists(_ element: Element) {
        if !self.contains(element) {
            self.append(element)
        }
    }
}

// Usage example
var myArray = [1, 2, 3]
myArray.appendIfNotExists(2) // Will not append since 2 already exists
myArray.appendIfNotExists(4) // Will append since 4 does not exist

print(myArray) // Output: [1, 2, 3, 4]