Yoreh Deah Siman 18 Seif 12

 

יב אַף עַל פִּי שֶׁבָּדַק הַסַכִּין קֹדֶם שְׁחִיטָה, צָרִיךְ לַחֲזֹר וְלִבְדֹּק אַחַר הַשְּׁחִיטָה; וְהָנֵי מִלֵּי שֶׁהַסַכִּין לְפָנֵינוּ, אֲבָל אִם נֶאֱבַד, שְׁחִיטָתוֹ כְּשֵׁרָה הוֹאִיל וּבְדָקָהּ קֹדֶם שְׁחִיטָה, וַאֲפִלּוּ שָׁחַט בָּהּ הַרְבֵּה זֶה אַחַר זֶה; וְאִם נָגַע בַּמַּפְרֶקֶת שֶׁל אַחַת מֵהֶן, יֵשׁ לְהַחְמִיר וְלָחוּשׁ לְכָל אוֹתָן שֶׁנִּשְׁחֲטוּ אַחַר כָּךְ

 

Seif 12

Although one checks the knife before the slaughter one needs to perform a check after the slaughtering.

 

This is talking about when he still has the knife, but if the knife got lost, the shechitah is kosher as long as the knife was checked before the shechitah, even if he slaughtered many one after the other.  If a damage was felt in the neck of one of them, one should be strict and worry about all those slaughtered afterwards.