What happens when both a Diablo 3 key and a Collector's Edition key are activated on one Battle.net account?

Solution 1:

You will be given the product code of your original license to use with a different account.

Blue Post

If you purchased Diablo III Standard Edition and you upgrade it to the Collector's Edition, a Diablo III Standard Edition key may be sent to a battle.net account of your friend.

Solution 2:

You will overwrite your D3 standard edition with your CE. Afterwards, you can contact Blizzard's support department - they state that in most situations you will get a new key for a D3 standard edition e.g. to give away.

See this blue post:

(...) Alternatively, you can get the digital version now, play right at midnight, overwrite your license with your CE when you pick it up, and then contact our support department. In most situations they'll be able to grant you a new standard key, which you can then give to a friend, or whatever.