Image test idiff.py acceptance not working as intended

[trexfeathers]

🐛 Bug Report

Thanks @rcomer for spotting this 🙂

Have looked with @bjlittle and we expect this is a hangover from the old idiff, where the duplicate check was for URI's. It's now for file names, and duplicates are expected during the acceptance process.

How To Reproduce

Steps to reproduce the behaviour:

1. Change any graphics test (anything that is a subclass of tests.GraphicsTest) to produce a different image from what's expected.
2. Follow the Adding or Updating Graphics Tests steps, clicking Accept in the idiff window when it appears.
3. You should get a message like this: DUPLICATE: result-gallery_tests.test_plot_COP_maps.0.png -> gallery_tests.test_plot_COP_maps.0.png (ignored)

Expected behaviour

Should instead get a ACCEPTED: ... message instead, with the image_repo.json file getting updated and the PNG file being renamed.

Screenshots

Environment

• OS & Version: [e.g., Ubuntu 20.04 LTS]
• Iris Version: [e.g., From the command line run python -c "import iris; print(iris.__version__)"]

Iris releases since v3.3.

Additional context

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[rcomer]

I just ran the Iris tests against Matplotlib and Cartopy main branches and saw a lot of image comparison failures. Both packages are likely to release fairly soon so it might be worth bumping this issue up the priority list.