Double Email Notification on Lock Screen and GMail?

Hi. The following problem: I have a Huawei P20 Pro. Have set up on the device via GMail two e-mail addresses. Both are @ t-online.de addresses and second is an additional address to the first. Now I get every email twice in the lock screen notified or displayed, whether it is at address 1 or address 2. Also in the app the messages are displayed twice. It was not like that on iOS before. Since I had set up both e-mail addresses. What could that be? A Weitetleitung or similar I did not furnish. Hope for help.

Do you have a main address and the possibility for one or more additional addresses?

Are you @ t-online

as an addition then daughter@t.online

etc.

Then the additional addresses are already forwarded directly from the t-online server to the main address. And set up by you again from Gmail. Power twice