fix importing creates duplicate resources

[huuphat1908]

Current logic

• Always generate new id for resources when importing
• No check for existing resources

New logic

• Only generate new id for resources when importing if the resource has no id
• Add logic to check if resources have already existed

Test data

• 5946-original-config.json
• 5946-original-config-with-additional-request.json

Test evidence

• Before: https://drive.google.com/file/d/1lxiVQCxYQvzTlecmDBDAGc7BZdLh_xN_/view?usp=sharing
• After: https://drive.google.com/file/d/1VkfwnGxU3CgMJwJrwzc2rVsxz2Dj0kRx/view?usp=sharing

Related issue: https://github.com/Kong/insomnia/issues/5946

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[TiagoGouvea]

This fix is highly relevant for us!

[gatzjames]

Thanks for the PR! Closing as this creates other issues on the UX and will be addressed in a separate PR.