This has been discussed at length in other forums--try searching in the the United forum (where you might have gotten more info if you had posted there).

In sum, Chase's marketing dept. does not appear to check to see if you have one or more UAL cards before you are mailed many offers for other cards. And, it you already have a card or two you are very unlikely to get the promised bonus miles if you bite on the mailed offers as the fine print says bonus miles are one time only.