Clean up some methods in dual module

[brynpickering]

Suggestions to:

• improve maintainability by spinning out repeated code into reusable functions
• reduce memory overhead by avoiding creating flattened arrays where possible

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

@bobbyxng pinging you on this :)

[bobbyxng]

Just got back from holidays, sorry for the late feedback! @FabianHofmann @brynpickering
Thanks a lot @brynpickering for the improvements, this looks way more efficient. I tested this branch and everything still works as it's supposed to, so I will merge your changes into the dual branch.