I have an old MacBook Air, which, when using Thunderbolt cable, I am able to mirror the screen on my iMac.
However, I cannot do the same for my newer MacBook 12".
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Ensure that under
System Preferences > Displays > Arrangement (this tab only shows up if your computer detects an external display), the "Mirror Displays" checkbox at the bottom is checked.
This checkbox determines whether your secondary display will act as an "extension" or as a mirror of your primary.