Akregator and Kmail under Kontact (version 5.5.0) open links twice in external browser when using left mouse button configuration. Links are nevertheless opened correctly if Akregator and Kmail are started as independent applications. This behavior started when KDE Applications were updated from 16.12 to 17.04. Things worked just fine in my system before the update. I am using KDE neon User Edition. Weirdly enough, if I change the config to left mouse button opens in a (background) tab, then it opens the link in a tab as well as in the external web browser. I tried removing the config files of Akregator, Kmail and Kontact, but this didn't fix the issue. Btw, middle mouse button in any configuration fails to work (and sometimes even makes Kontact crash), please let me know if I should fill another bug for this issue. I'm willing to help developers to identify the root problems here. Please let me know if there is anything I could test or try to get this solved or whether there is a way to collect more info. Thanks!
Disabling phishing checks in Akregator and Kmail workarounds the issue for now until it gets fixed. Thanks dvratil for finding this out! However, the issue on the middle mouse button seems to be totally unaffected, so I guess I should fill another bug for it...