How do I fix my double tapped outside electric panel?
To simply look at this situation and immediately call it a violation, is incorrect.  While it may indeed be a violation, more investigation is required.
At the end of section 110.14(A) in the National Electrical Code (NEC), there is this short paragraph.

NEC 2008
110.14 Electrical Connections.
(A) Terminals. ...Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.

Which means if the terminal allows multiple conductors, it should be labeled somewhere either on the terminal itself or in the documentation inside the panelboard.  So make sure you read all of the labels inside the panelboard, before marking this situation as a violation.
However, Section 408.41 of the NEC, clearly says that the grounded (neutral) terminal block is a violation.

408.41 Grounded Conductor Terminations. Each grounded conductor shall terminate within the panelboard in an individual terminal that
is not also used for another conductor.

There is an exception to this section, but it does not apply in this situation.
But wait, there's more...  Section 90.4 of the NEC, allows the Authority Having Jurisdiction (AHJ) to waive requirements or permit alternative methods.

90.4 Enforcement. ...The authority having jurisdiction for enforcement of the Code has the responsibility for making
interpretations of the rules, for deciding on the approval of
equipment and materials, and for granting the special permission
contemplated in a number of the rules.
By special permission, the authority having jurisdiction may waive
specific requirements in this Code or permit alternative methods where
it is assured that equivalent objectives can be achieved by
establishing and maintaining effective safety.

There is also 90.2(C), which may fit your situation better.

90.2(C) Special Permission. The authority having jurisdiction for enforcing this Code may grant exception for the installation of
conductors and equipment that are not under the exclusive control of
the electric utilities and are used to connect the electric utility
supply system to the service entrance conductors of the premises
served, provided such installations are outside a building or
terminate immediately inside a building wall.

It may also be important to note, that the installation is only subject to be compliant with the currently adopted code.  Which means if this was installed before these codes were adopted, this could be a legitimate installation.
The best thing for you to do, would be to contact a local licensed Electrician. Ask them to take a look, and make recommendations as to how this situation should be handled.  If you contact the local electrical inspectors office. They should be able to tell you when the work was completed and approved, and by whom. They should also be able to find any special notes supplied by the inspector, that would indicate if the inspector gave special permission in this situation.  If the work was completed by an unlicensed Electrician, the homeowner may be responsible for the repairs. But this would completely depend on your location, as well as many other legal factors that I will not discuss here.
Fixing this is not a DIY task.