<?php

$arr = ['6920709201700750550', '6920702500550400', '69207000550400', '7740641201800750550', '7740642500550400',
'79378270201900750550', '7937822500550400', '79575693201900750550', '7957562500550400', '79575600550400'];

$novo = [];

foreach($arr as $item){
	preg_match('/^\d{6}/', $item, $m);
	if(!in_array($m[0], $novo)){
		$novo[] = $m[0];
	}
}

print_r($novo);