Any help with this is hugely appreciated.

My band have been asked to have one of our tracks included in an upcoming feature film soundtrack. As part of this, the producer is asking for proof of copyright ownership. The track is part of a release that we registered through both Tunecore and CDBaby. During the process, I also tried to register the ISRCs through the PPL, but they said there was a duplicate claim already made. Since at the time it was just our closest friends who knew about the music we were making, I had assumed that the duplicate claim was due to registering it through other services. I've had a look through those sites and can't find anything meaningful there which states that the copyright claim is ours.

The producer has warned us that we should get this registered with the Electronic Copyright Office (eCO) in the USA as soon as possible, but I'm just wondering if this is necessary.

Is there any way I can find out whether or not I have registered the copyright on the tracks we've released?

Joke replies also welcome as they'll bump this and make it more likely to be seen by someone who may know something about this.