Duplicate files · Scanning and Auto-scan
Why does the Duplicates window start scanning as soon as I open it?
Short answer: With Auto-scan on, opening a source whose results are more than a few minutes old refreshes it so you see current results. Click Stop to end it, or switch Auto-scan off for that source to only scan when you click Scan now.
When Auto-scan is on for a source and its results are more than a few minutes old, opening that source refreshes it in the background so you see current results — with the same progress line as a manual scan. Clicking between sources doesn't re-scan them over and over, and anything already scanning is left alone.
If you'd rather it didn't, click Stop, or turn Auto-scan off for that source; the window will then just show the last results until you click Scan now.