I2C library problem in nRF51-dk , and importing failure

I have a project using DS1631 I2C temperature sensor. The project complied fine in August, but using the latest mbed OS, I got the following message:
Error: Symbol cmd multiply defined (by …/…/build/mbed-os/features/frameworks/mbed-client-cli/source/ns_cmdline.NRF51_DK.o and …/…/build/DS1631/DS1631.NRF51_DK.o).

I think there could be a bug in the I2C library of mbed-os for nRF51-dk board.

By the way, I was trying to bypass the problem by exporting the project, and importing it back for someone else to use. I exported using the .zip format, but when I imported it back, I see the following error:
Error: Invalid import type.