That's an interesting issue! I tried a few things and I'm not seeing this, so I have a few questions about your setup:
1. What version of Snagit 2020 and macOS are you running? 2. Are you using a capture preset or just the big red button? 3. Are you actually getting 2 captures appearing in Snagit's editor? 4. What email client are you seeing this with? Does it happen if you paste into other apps, like maybe a word document? 5. Do you have a clipboard manager app that lets you see what's in your clipboard? If so, is it showing duplicates?
A little technical background for these questions if you're interested: when you copy something to clipboard it's standard to put a few different file formats there so each destination you paste into can decide what format it wants. Basically i'm trying to figure out if this is a bug with Snagit or with the destination you're pasting to.