CoralNet identifies images by their names when you upload metadata, so if it finds two images with the same name, it doesn't know what to do. Normally, CoralNet prevents a source's images from being uploaded/edited to have the same names, but I think it can still occasionally happen if operations like image uploads are done with a specific timing. It's likely a bug in the code that needs to be fixed sometime.
To stop the error, you'll want to either delete or rename one of the two images above.