Duplicate files · Scanning and Auto-scan
How do I turn off background duplicate scanning?
Short answer: Pick the source in the Duplicates sidebar and switch Auto-scan off in its header. Each source has its own switch; with it off, that source is only checked when you click Scan now. Status is also shown in Preferences → Services under Duplicate file finder.
Select the source in the Duplicates window's sidebar and switch Auto-scan off in its header. Each source has its own switch, so you can, for example, keep cloud accounts checked automatically but scan This Mac only when you ask.
With Auto-scan off, nothing is checked in the background for that source; click Scan now whenever you want fresh results. Your previous results stay visible until you scan again.
You can see whether the background finder is running at any time in Preferences → Services, under Duplicate file finder.