Currently we don’t have support to find the duplicate values using conditional format and we are currently working on this . These changes will be included in our main release 2019 Vol 1 which will be available in March 2019.
However, we can achieve your requirement as a workaround by using conditional format formula. We have prepared a sample for the same which can be downloaded from the following link.
Kindly try this and let us know if this helps.