Example string concatenate formula:

WhilePrintingRecords;
StringVar Donors;
if Donors= "" then Donors := {CnBio.CnBio_Name}
else Donors := Donors & ", " & {CnBio.CnBio_Name}

Example string concatenate formula with the inStr() function:

WhilePrintingRecords;
StringVar Donors;
if Donors = "" then Donors:= {CnBio.CnBio_Name}
else if inStr(Donors, {CnBio.CnBio_Name}) = 0 then Donors := Donors & ", " & {CnBio.CnBio_Name}