Hello,
when I klick on a link in Akregator as part of Kontact, the Webbrowser opens the link twice. If I open the same link in Akregator, started as single app, it opens only one Browser tab. This happend when I klck on a link inside a article, a double klick on the headline will open a single tab.
When configure a different browser in the settings of Akregator, one tab in the stanard browser opens and one in the browser configured in Akregator.
This also happend in Kmail, as a part of Kontact links are open twice, Kmail as a single app it works as expected.

Is there a setting in Kontact, to only have one tab open in the Browser?

I can't reproduce this on my Akregator install. What distribution and Akregator version are you using?

I'm using KDE Neon 5.10 (User Edition), Akregator Version 5.5.2, Kontact Verison 5.5.2

Sometimes I have a similar issue: I open a link in an email from kmail (within kontact) and the link opens in the webbrowser as expected, but also sometimes as a tab in akregator.

(Latest openSUSE Tumbleweed)

It would be best to file a bug on bugs.kde.org. Also, please check if you have Qt 5.9 installed, or an earlier version.

Today, Neon shipped an update to KDE-Plasma 5.10.4, and the strange behavior is gone!