I've received my order twice. And have only been charged for the first. Can I legally keep the second item?

Technically no you cannot keep the second. Depending on your country of domicile there will be a range of laws that make doing so "theft. However, in the UK if you contact the supplier and inform them of the "oversight" it becomes THEIR responsibility to collect the item within a fair and reasonable time span - if they do not then the items, because they are unsolicited are yours.

Depending on the value of the item concerned many companies have to balance the total cost of the item against what it will cost to arrange collection and return.

Using the adage "honesty is the best policy" could ensure that you are NOT charged for 2 of the same item.