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Finding duplicate files – with Windows tools and with Zortio

How to track down duplicate files in Windows: what Explorer can do, why comparing names misleads you, and how a content comparison works.

Duplicate files appear almost on their own. You download the same invoice twice, copy a folder “just to be safe”, and eventually the same holiday photos sit in three places. Disk space is one problem – the bigger one is that you no longer know which version is the real one.

Why Explorer alone is not enough

Windows Explorer can sort by size, and that is genuinely a useful first step: two files with exactly the same byte count are suspicious. Open the folder, switch to the details view and click the “Size” column.

The approach has two gaps though. First, plenty of unrelated files happen to share a size – small text files especially. Second, you only find what sits side by side; across several folders it gets unmanageable fast.

The mistake almost every tool makes

Many cleanup tools compare file names. They treat “Invoice.pdf” and “Invoice (1).pdf” as the same thing. Sometimes that is right. Often though, “(1)” is a newer, corrected version – and then you delete the wrong one.

How a content comparison works

Only comparing the content is reliable. A checksum is calculated for each file – a kind of fingerprint. If two fingerprints match, the content matches, regardless of the name.

Reading every file in full would be slow, however. Zortio therefore works in three stages and only reads where it pays off:

1. Compare the size. That costs nothing and rules out the vast majority of pairs immediately.
2. Compare the first 128 KB. That drops files that merely happen to be the same size.
3. Only what is still left gets read and compared in full.

Whatever survives those three stages really is identical byte for byte. No guessing, no similarity – equality.

Which of the two files should stay?

Once two files are identical, the question is no longer which one is “right” – the content is the same. The question is which name and which place should remain. Zortio prefers the file without a copy marker in its name, and the older one when that ties. So out of “Invoice.pdf” and “Invoice (2).pdf”, it is “Invoice.pdf” that stays.

Delete? Better not straight away

The most important advice last: never delete something a tool proposes as a duplicate without looking yourself. Zortio therefore deletes nothing at all and instead moves duplicates into a folder called “Duplicates – can go”. You see at a glance what has accumulated and decide yourself – or undo the whole run with one click.