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function subm(f,newtarget)
{
document.myform.target = newtarget ;
f.submit();
}
<FORM name="myform" method="post" action="" target="" >
<INPUT type="button" name="Submit" value="Submit" onclick="subm(this.form,'_self');">
<INPUT type="button" name="Submit" value="Submit" onclick="subm(this.form,'_blank');">