Dollar bulls see Donald Trump's plan to repatriate as much as $2.6 trillion in corporate profits stashed overseas as a can't-miss boon for the greenback. The problem is most of that hoard may already be held in the U.S. currency.

Traders pushed the dollar up by the most in five years last week, partly in a wager that the president-elect's tax-amnesty proposals will fuel a surge in greenback demand.

Yet at Apple Inc., with the most overseas cash among S&P 500 members, more than 90% of its $216 billion stash is in the U.S. currency, according to former employees who had direct knowledge of the matter and asked not to be identified. For Microsoft Corp., the second-largest holder of cash abroad, dollar-denominated bonds alone make up 66% of total cash, securities filing shows. The data suggest that even if Trump is able to follow through with a tax amnesty similar to the one-time, 10% holiday he's proposed, its effects on the world's reserve currency would be minimal.