So reporting as ordinary dividend was taxed once and the return of capital reduces the cost basis (= capital gain) which will be taxed again when you sell?

@wanghsiaochih 

If you mistakenly reported a non dividend distribution, as  ordinary income, in the past, and paid tax on it; then you do NOT reduce your cost basis, for the future sale.

Alternatively, you can file amended returns (going back not more than three years) and correctly report the  non dividend distribution. In that case, you would reduce your cost basis.  Either way, you will not be taxed twice.