Short term, high interest, predatory loans secured by personal property and threats of violence.
Fund a crooked politician's election campaign in exchange for business to my own little corporation which would provide minimal service at high expense to the taxpayer.
Fund a coup in a developing nation (probably requires other investors as well), then exploit the local people and resources.
Ponzi scheme.
Land contracts, get half payment up front, file for bankruptcy, leave town, new alias, repeat.
Form a mercenary group to fight alongside the Assad government, loot massive quantities of weapons and oil, leave.
None of these are, of course, particularly ethical mans of doubling your capital, but I doubt that there's much you can ethically do to meet the goal in just a year.