COUNTIF inconsistency

I came across a great tip for finding duplicates in long data lists (mrexcel.com/tip138.shtml), but its only working some of the time.

The formula:

=COUNTIF(C$1:C1,C2)=0

1. I have long number strings in 5,000 row spreadsheets
2. I'm trying to find duplicates in numbers that look like this: "014824-00242"
3. The COUNTIF command will sometimes display accurately, sometimes list everything as "TRUE" regardless of the content. Sometimes it does.

Screencap:
\1

Attempted workarounds:

• I have pasted into a fresh CSV spreadsheet to try and get any odd formatting out, then saving back into an XLSX file.
• Also, restarting Excel seem to do the trick, before but it doesn't anymore.

Does anyone have any idea what could be causing this? Its driving me crazy that it was one moment solving problems and the next ignoring my data.

Thanks!