First of all you are not using the COUNTIF funcion correctly. You should use
a conditional sentence inclosing it..sthing like.
=if(COUNTIF('sheet1'!AB2:AB210,G2)0);"Duplicate"; "Not Duplicate")
this is one way... theoretically it should work.
Whenever i have this problem i use instead:
=if(iserror(vlookup('sheet1'!AB2:AB210;G2;0));"Not Duplicate";"Duplicate")
Hope this helps,

Albion - Argentina

"AChesley" escribió en el mensaje
...
I am trying to compare the data between two sheets to locate matches. I
have
attempted the Countif, Or(exact) and =ISNA(Match) - they all produce
results..but when checking..the results are innaccurate.

For example...Comparing Unigue ID on Sheet 1 to Unigue ID on Sheet 2.

If I use =COUNTIF('sheet1'!AB2:AB210,G2)0
I get a value of False in all rows...

What should I be using to obtain this information?

Thanks!