New messages getting delievered to two phones after restoring a backup I took a backup of my data on my iPhone 6s and then restored it on an iPhone 7.
Now there is another SIM in the 6s, but all messages coming in on the 6s are also coming on the 7.
You probably used the same Apple-ID for the new phone, and then all the iMessages get forewarded.

If you want to have a closer look, here is a good explanation of how it works:
selfsolve.apple.com

in short, you can do the following steps, to separate the messages on the two devices:


Transfer your SIM card to your old iPhone.
Go to Settings
Tap Messages
Turn iMessage off
Go back to Settings
Tap FaceTime
Turn FaceTime off