change formula without validation data

You can use:

=IF(B11="","",IF(ISNA(VLOOKUP(B11,Foglio1!$A$2:$B$1500,2,FALSE)),"codice non presente in articoli o errato!",IF(COUNTIF($B$2:$B$1500,$B11)>1,"Value " & $B11 & " at " & INDEX($A$2:$A$1500,MATCH($B11,$B$2:$B$1500,0)),VLOOKUP(B11,Foglio1!$A$2:$B$1500,2,FALSE))))

in column C, but data validation in column B wont allow dublicates.