=LET(comparelookup,
   LET(
     matchedstring,XLOOKUP([@number],mytable[number],mytable[compared2],"number not found",0),
     compareleft,LEFT(matchedstring,20),
     IF(compareleft="string to trim from left",MID(matchedstring,22,LEN(matchedstring)-21),matchedstring)
   ),
   IF(comparelookup=[@compared1],"dupe",comparelookup)
)