Natively there is not a setting to have a distinct using Rollup Helper.

However with this workaround it is possible!

First you will want to create a text Rollup that will concatenate all of the unique names for the records you want to count. Make sure under advanced options the checkbox for unique only is selected. Also make the delimiter for the field a semicolon.

Once this Rollup is complete you will need to create a formula field that will count the number of records in the concatenated list. Make the formula field return a number. Here is the formula you will want to use:  

IF( ISBLANK(name) , 0, LEN(name)-LEN(SUBSTITUTE(name,";",""))/LEN(";")+1)

Replace the word "name" with the name of field that contains your concatenated list of unique records.

Your Formula field will now populate with a unique count of the records!