The 15 digit version of Salesforce IDs can get mangled by excel as they can be all numbers - so (helpfully?) Excel will treat them as a date.
Also, Excel's "remove duplicates" is case insensitive, so actually-not-duplicated IDs may get removed (as the 14 digit version is case sensitive).
The 18 digit version solves both issues.
=A2 & LOOKUP(IF(AND(CODE(RIGHT(LEFT($A2,5),1))>=65,CODE(RIGHT(LEFT($A2,5),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,4),1))>=65,CODE(RIGHT(LEFT($A2,4),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,3),1))>=65,CODE(RIGHT(LEFT($A2,3),1))<=90),1,0)& IF(AND(CODE(RIGHT(LEFT($A2,2),1))>=65,CODE(RIGHT(LEFT($A2,2),1))<=90),1,0)& IF(AND(CODE(LEFT($A2,1))>=65,CODE(LEFT($A2,1))<=90),1,0),{"00000","00001","00010","00011","00100","00101","00110","00111","01000","01001","01010","01011","01100","01101","01110","01111","10000","10001","10010","10011","10100","10101","10110","10111","11000","11001","11010","11011","11100","11101","11110","11111"},{"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5"}) & LOOKUP(IF(AND(CODE(RIGHT(LEFT($A2,10),1))>=65,CODE(RIGHT(LEFT($A2,10),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,9),1))>=65,CODE(RIGHT(LEFT($A2,9),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,8),1))>=65,CODE(RIGHT(LEFT($A2,8),1))<=90),1,0)& IF(AND(CODE(RIGHT(LEFT($A2,7),1))>=65,CODE(RIGHT(LEFT($A2,7),1))<=90),1,0)& IF(AND(CODE(RIGHT(LEFT($A2,6),1))>=65,CODE(RIGHT(LEFT($A2,6),1))<=90),1,0),{"00000","00001","00010","00011","00100","00101","00110","00111","01000","01001","01010","01011","01100","01101","01110","01111","10000","10001","10010","10011","10100","10101","10110","10111","11000","11001","11010","11011","11100","11101","11110","11111"},{"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5"}) & LOOKUP(IF(AND(CODE(RIGHT(LEFT($A2,15),1))>=65,CODE(RIGHT(LEFT($A2,15),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,14),1))>=65,CODE(RIGHT(LEFT($A2,14),1))<=90),1,0) & IF(AND(CODE(RIGHT(LEFT($A2,13),1))>=65,CODE(RIGHT(LEFT($A2,13),1))<=90),1,0)& IF(AND(CODE(RIGHT(LEFT($A2,12),1))>=65,CODE(RIGHT(LEFT($A2,12),1))<=90),1,0)& IF(AND(CODE(RIGHT(LEFT($A2,11),1))>=65,CODE(RIGHT(LEFT($A2,11),1))<=90),1,0),{"00000","00001","00010","00011","00100","00101","00110","00111","01000","01001","01010","01011","01100","01101","01110","01111","10000","10001","10010","10011","10100","10101","10110","10111","11000","11001","11010","11011","11100","11101","11110","11111"},{"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z","0","1","2","3","4","5"})
