Update only if entry does not exist I have to update many records. I don't want to duplicate the entries.  The table concatenates the value to the end.  So in theory, I could add the value twice.

I want to put a check to make sure the entry is not already there; if it is, then skip the record.

update table1 
set value=concat(value,',test_data') 
where field1='abc';
UPDATE table1 
SET value=CONCAT(value, ',test_data') 
WHERE field1='abc'
  AND NOT INSTR(CONCAT(',',value,','), ',test_data,');


But I'd recommend to normalize your data structure... CSV data field is not safe.