CountIf Including String

I have a column of IDs and I want to check if there are duplicates in this column as I loop through it.
However, I need to ignore suffixes to the data.
e.g.

Code:

---

123 - false
124 - true
124 - true
125 - true (matches 125_1 and 125_2)
125_1  - true (matches 125 and 125_2)
125_2  - true (matches 125 and 125_1)
126_1  - true (matches 126_2)
126_2  - true (matches 126_1)
127 - false

---

Can I use CountIfs for this?
Or maybe multiple CountIf's.

I'm concerned that if I have to write it myself in VBA it wont be as efficient as the built in functions

This is the most simplest way to do it...

1) Sort the column in ascending order (Say the data is from A1 and down)
2) Type this formula in B2 and copy it down

=IF(LEFT(A2,3)=LEFT(A1,3),"True","False")

Hope this helps...