From within a campaign, click Goals at the top of your screen.

From the goals page, you will see the button "Create Goal" in the top right-hand corner.

Click this button and you will be presented with a screen asking you to choose if you wish to create a new goal or clone an already existing one, choose "Clone Goal".

Next, you will be asked to choose which custom goal you would like to clone. Here you can choose the campaign that contains the custom goal you would like to clone.
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Select the custom goal to clone, then click "Select goal".

Next, you will be able to edit your custom goal. Note that these edits will only apply to the clone you've just created. You can change the name, interval, description, metrics, conditions and values of the cloned custom goal.

When you're done, click "Create Goal" and your cloned custom goal will be complete.

Once you're done, you can add your custom goal to any report or custom dashboard via the usual "add widget" process.

Note: If you see "an error occurred" after saving your widget, please check that your custom goal is valid and that the integrations relating to your custom goal are connected properly.