When you sell the land/plot, you can claim exemption u/s 54F only by investing the sale proceeds in the construction of a new house. The exemption is, however, not available, if "the assessee,—
(i) owns more than one residential house, other than the new asset, on the date of transfer of the original asset;" as per the proviso to 54F(1).
As you are already having one one-floor residential house, you will not be entitled to claim exemption u/s 54F for the construction of your duplex house.
However, if your present one floor house is constructed within one year before the date of sale, then you can claim exemption for the existing house also.