help with duplicate flag in string functions

Hi, I'm just starting out writing modules by reading api docs on zend.com and reading source code of existing modules. Several functions (example:) add_next_index_string(zval *array, char *str, int duplicate) have the int duplicate flag, the docs say, The flag duplicate specifies whether the string contents have to be copied to Zend internal memory. but how do I know if the string should be copied to memory, in the simple functions I'm writing it seams to make no difference whether I set this to 1 or 0 but it must do something? Thanks for your time Tony