Create the following three formulas in the Crystal Report:

Formula 1: this formula concatenates each value into one string. Place in Details section and suppress the field.
WhilePrintingRecords;
Shared StringVar ConCat;
If ConCat = "" then ConCat := {Field_Name} else if not({Field_Name} in ConCat) then ConCat := ConCat + ", " + {Field_Name}

Formula 2: this formula will display the final value. Place in Group Footer section.
WhilePrintingRecords;
Shared StringVar ConCat;

Formula 3: this formula will reset the variable for the next record. Place in Group Header to remove previous record's value.
WhilePrintingRecords;
Shared StringVar ConCat := ""