Create SSRS variable from SQL table records

I need to create a variable in my SQL stored procedure from records in a table, and when finished will look like:

2015-10-20,2015-10-21,2015-10-22,2015-10-23      etc...

DECLARE @mDates VARCHAR(max)

I want the variable to mimic what a actual SSRS multi-select parameter looks like when it is inputted into a stored procedure and not contain duplicates, which is a possibility.

Lets say TABLEA contains these records (could be more could be less could be just 1 record):    (max is a 30 records in this table)

  2015-10-20
  2015-10-21
  2015-10-22
  2015-10-23