doubled RMAN files

We're using same script and setup for multiple databases, and for some reason one of the databases backup is now doing this.

A full backup is some 400Gig

When we run this now, only on that database, he's creating TWO such files, two times a 400Gig files instead of one.

The databases are copies of each other, the other databases create a backup file (one) of the same size (400Gig).

But on this one database, since a specific point in time, he's creating two files. They are identical on size, to the byte. They are also created at exactly the same time, in the same RMAN session. There's no double backup session running, all happens in the same session.