string members = '001C000001DgWjDIAV,001C000001DgWjEIAV,001C000001DgWjDIAV';

set<string> memberIds = new set<string>();
for (String str :members.split(',')){
    memberIds.add(str);
}

String uniquemembers = JSON.serialize(memberIds).replaceAll('["\[\]]','');
system.debug(members);
system.debug(uniquemembers);

23:58:32:003 USER_DEBUG [9]|DEBUG|001C000001DgWjDIAV,001C000001DgWjEIAV,001C000001DgWjDIAV
23:58:32:003 USER_DEBUG [10]|DEBUG|001C000001DgWjDIAV,001C000001DgWjEIAV

String csv = '001C000001DgWjDIAV,001C000001DgWjEIAV,001C000001DgWjDIAV';
List<String> l = new List<String>(new Set<String>(members.split(',')));
String uniqueCsv = String.join(l, ',');