Constant ADODB_PREFETCH_ROWS already defined

A Notice: "Constant ADODB_PREFETCH_ROWS already defined" error occurs, already on the installation wizard pages. This happens in setuplib.php, row 297: define ('ADODB_PREFETCH_ROWS', 1000);

I can solve the problem by change into: if (!defined('ADODB_PREFETCH_ROWS')) define ('ADODB_PREFETCH_ROWS', 1000);
But of course i'd like to know why this error occurs in the first place